diff --git a/composer.json b/composer.json
index 27cd7d9af..2d277b84f 100644
--- a/composer.json
+++ b/composer.json
@@ -28,6 +28,7 @@
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index 6a9ae7127..3d552f874 100644
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-                "reference": "c1c2e997aa3146983ed888ad08b15470a2e22ecc"
+                "reference": "e1e4a170560925c26d424b6a03aed157e7dcc5c5"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/lines-of-code/zipball/c1c2e997aa3146983ed888ad08b15470a2e22ecc",
-                "reference": "c1c2e997aa3146983ed888ad08b15470a2e22ecc",
+                "url": "https://api.github.com/repos/sebastianbergmann/lines-of-code/zipball/e1e4a170560925c26d424b6a03aed157e7dcc5c5",
+                "reference": "e1e4a170560925c26d424b6a03aed157e7dcc5c5",
                 "shasum": ""
             },
             "require": {
-                "nikic/php-parser": "^4.6",
+                "nikic/php-parser": "^4.18 || ^5.0",
                 "php": ">=7.3"
             },
             "require-dev": {
@@ -2063,7 +2097,7 @@
             "homepage": "https://github.com/sebastianbergmann/lines-of-code",
             "support": {
                 "issues": "https://github.com/sebastianbergmann/lines-of-code/issues",
-                "source": "https://github.com/sebastianbergmann/lines-of-code/tree/1.0.3"
+                "source": "https://github.com/sebastianbergmann/lines-of-code/tree/1.0.4"
             },
             "funding": [
                 {
@@ -2071,7 +2105,7 @@
                     "type": "github"
                 }
             ],
-            "time": "2020-11-28T06:42:11+00:00"
+            "time": "2023-12-22T06:20:34+00:00"
         },
         {
             "name": "sebastian/object-enumerator",
@@ -2479,16 +2513,16 @@
         },
         {
             "name": "squizlabs/php_codesniffer",
-            "version": "3.7.2",
+            "version": "3.8.1",
             "source": {
                 "type": "git",
-                "url": "https://github.com/squizlabs/PHP_CodeSniffer.git",
-                "reference": "ed8e00df0a83aa96acf703f8c2979ff33341f879"
+                "url": "https://github.com/PHPCSStandards/PHP_CodeSniffer.git",
+                "reference": "14f5fff1e64118595db5408e946f3a22c75807f7"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/squizlabs/PHP_CodeSniffer/zipball/ed8e00df0a83aa96acf703f8c2979ff33341f879",
-                "reference": "ed8e00df0a83aa96acf703f8c2979ff33341f879",
+                "url": "https://api.github.com/repos/PHPCSStandards/PHP_CodeSniffer/zipball/14f5fff1e64118595db5408e946f3a22c75807f7",
+                "reference": "14f5fff1e64118595db5408e946f3a22c75807f7",
                 "shasum": ""
             },
             "require": {
@@ -2498,11 +2532,11 @@
                 "php": ">=5.4.0"
             },
             "require-dev": {
-                "phpunit/phpunit": "^4.0 || ^5.0 || ^6.0 || ^7.0"
+                "phpunit/phpunit": "^4.0 || ^5.0 || ^6.0 || ^7.0 || ^8.0 || ^9.3.4"
             },
             "bin": [
-                "bin/phpcs",
-                "bin/phpcbf"
+                "bin/phpcbf",
+                "bin/phpcs"
             ],
             "type": "library",
             "extra": {
@@ -2517,22 +2551,45 @@
             "authors": [
                 {
                     "name": "Greg Sherwood",
-                    "role": "lead"
+                    "role": "Former lead"
+                },
+                {
+                    "name": "Juliette Reinders Folmer",
+                    "role": "Current lead"
+                },
+                {
+                    "name": "Contributors",
+                    "homepage": "https://github.com/PHPCSStandards/PHP_CodeSniffer/graphs/contributors"
                 }
             ],
             "description": "PHP_CodeSniffer tokenizes PHP, JavaScript and CSS files and detects violations of a defined set of coding standards.",
-            "homepage": "https://github.com/squizlabs/PHP_CodeSniffer",
+            "homepage": "https://github.com/PHPCSStandards/PHP_CodeSniffer",
             "keywords": [
                 "phpcs",
                 "standards",
                 "static analysis"
             ],
             "support": {
-                "issues": "https://github.com/squizlabs/PHP_CodeSniffer/issues",
-                "source": "https://github.com/squizlabs/PHP_CodeSniffer",
-                "wiki": "https://github.com/squizlabs/PHP_CodeSniffer/wiki"
+                "issues": "https://github.com/PHPCSStandards/PHP_CodeSniffer/issues",
+                "security": "https://github.com/PHPCSStandards/PHP_CodeSniffer/security/policy",
+                "source": "https://github.com/PHPCSStandards/PHP_CodeSniffer",
+                "wiki": "https://github.com/PHPCSStandards/PHP_CodeSniffer/wiki"
             },
-            "time": "2023-02-22T23:07:41+00:00"
+            "funding": [
+                {
+                    "url": "https://github.com/PHPCSStandards",
+                    "type": "github"
+                },
+                {
+                    "url": "https://github.com/jrfnl",
+                    "type": "github"
+                },
+                {
+                    "url": "https://opencollective.com/php_codesniffer",
+                    "type": "open_collective"
+                }
+            ],
+            "time": "2024-01-11T20:47:48+00:00"
         },
         {
             "name": "symfony/polyfill-php73",
@@ -2677,16 +2734,16 @@
         },
         {
             "name": "theseer/tokenizer",
-            "version": "1.2.1",
+            "version": "1.2.2",
             "source": {
                 "type": "git",
                 "url": "https://github.com/theseer/tokenizer.git",
-                "reference": "34a41e998c2183e22995f158c581e7b5e755ab9e"
+                "reference": "b2ad5003ca10d4ee50a12da31de12a5774ba6b96"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/theseer/tokenizer/zipball/34a41e998c2183e22995f158c581e7b5e755ab9e",
-                "reference": "34a41e998c2183e22995f158c581e7b5e755ab9e",
+                "url": "https://api.github.com/repos/theseer/tokenizer/zipball/b2ad5003ca10d4ee50a12da31de12a5774ba6b96",
+                "reference": "b2ad5003ca10d4ee50a12da31de12a5774ba6b96",
                 "shasum": ""
             },
             "require": {
@@ -2715,7 +2772,7 @@
             "description": "A small library for converting tokenized PHP source code into XML and potentially other formats",
             "support": {
                 "issues": "https://github.com/theseer/tokenizer/issues",
-                "source": "https://github.com/theseer/tokenizer/tree/1.2.1"
+                "source": "https://github.com/theseer/tokenizer/tree/1.2.2"
             },
             "funding": [
                 {
@@ -2723,7 +2780,7 @@
                     "type": "github"
                 }
             ],
-            "time": "2021-07-28T10:34:58+00:00"
+            "time": "2023-11-20T00:12:19+00:00"
         },
         {
             "name": "wp-coding-standards/wpcs",
@@ -2793,7 +2850,7 @@
         },
         {
             "name": "wp-phpunit/wp-phpunit",
-            "version": "5.9.7",
+            "version": "5.9.8",
             "source": {
                 "type": "git",
                 "url": "https://github.com/wp-phpunit/wp-phpunit.git",
@@ -2907,6 +2964,7 @@
     "prefer-lowest": false,
     "platform": {
         "php": ">=7|^8",
+        "ext-dom": "*",
         "ext-json": "*"
     },
     "platform-dev": [],
diff --git a/modules/images/image-loading-optimization/class-ilo-html-tag-processor.php b/modules/images/image-loading-optimization/class-ilo-html-tag-processor.php
index f42bdce21..0a69d25c2 100644
--- a/modules/images/image-loading-optimization/class-ilo-html-tag-processor.php
+++ b/modules/images/image-loading-optimization/class-ilo-html-tag-processor.php
@@ -132,6 +132,8 @@ public function __construct( string $html ) {
 	 * A generator is used so that when iterating at a specific tag, additional information about the tag at that point
 	 * can be queried from the class. Similarly, mutations may be performed when iterating at an open tag.
 	 *
+	 * @since n.e.x.t
+	 *
 	 * @return Generator<string> Tag name of current open tag.
 	 */
 	public function open_tags(): Generator {
@@ -191,7 +193,11 @@ public function open_tags(): Generator {
 				yield $tag_name;
 
 				// Immediately pop off self-closing tags.
-				if ( in_array( $tag_name, self::VOID_TAGS, true ) ) {
+				if (
+					in_array( $tag_name, self::VOID_TAGS, true )
+					||
+					( $p->has_self_closing_flag() && $this->is_foreign_element() )
+				) {
 					array_pop( $this->open_stack_tags );
 				}
 			} else {
@@ -200,32 +206,21 @@ public function open_tags(): Generator {
 					continue;
 				}
 
-				// Since SVG and MathML can have a lot more self-closing/empty tags, potentially pop off the stack until getting to the open tag.
-				$did_splice = false;
-				if ( 'SVG' === $tag_name || 'MATH' === $tag_name ) {
-					$i = array_search( $tag_name, $this->open_stack_tags, true );
-					if ( false !== $i ) {
-						array_splice( $this->open_stack_tags, $i );
-						$did_splice = true;
-					}
-				}
-
-				if ( ! $did_splice ) {
-					$popped_tag_name = array_pop( $this->open_stack_tags );
-					if ( $popped_tag_name !== $tag_name && function_exists( 'wp_trigger_error' ) ) {
-						wp_trigger_error(
-							__METHOD__,
-							esc_html(
-								sprintf(
-									/* translators: 1: Popped tag name, 2: Closing tag name */
-									__( 'Expected popped tag stack element %1$s to match the currently visited closing tag %2$s.', 'performance-lab' ),
-									$popped_tag_name,
-									$tag_name
-								)
+				$popped_tag_name = array_pop( $this->open_stack_tags );
+				if ( $popped_tag_name !== $tag_name && function_exists( 'wp_trigger_error' ) ) {
+					wp_trigger_error(
+						__METHOD__,
+						esc_html(
+							sprintf(
+								/* translators: 1: Popped tag name, 2: Closing tag name */
+								__( 'Expected popped tag stack element %1$s to match the currently visited closing tag %2$s.', 'performance-lab' ),
+								$popped_tag_name,
+								$tag_name
 							)
-						);
-					}
+						)
+					);
 				}
+
 				array_splice( $this->open_stack_indices, count( $this->open_stack_tags ) + 1 );
 			}
 		}
@@ -236,6 +231,8 @@ public function open_tags(): Generator {
 	 *
 	 * A breadcrumb consists of a tag name and its sibling index.
 	 *
+	 * @since n.e.x.t
+	 *
 	 * @return Generator<array{string, int}> Breadcrumb.
 	 */
 	private function get_breadcrumbs(): Generator {
@@ -244,12 +241,30 @@ private function get_breadcrumbs(): Generator {
 		}
 	}
 
+	/**
+	 * Determines whether currently inside a foreign element (MATH or SVG).
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @return bool In foreign element.
+	 */
+	private function is_foreign_element(): bool {
+		foreach ( $this->open_stack_tags as $open_stack_tag ) {
+			if ( 'MATH' === $open_stack_tag || 'SVG' === $open_stack_tag ) {
+				return true;
+			}
+		}
+		return false;
+	}
+
 	/**
 	 * Gets XPath for the current open tag.
 	 *
 	 * It would be nicer if this were like `/html[1]/body[2]` but in XPath the position() here refers to the
 	 * index of the preceding node set. So it has to rather be written `/*[1][self::html]/*[2][self::body]`.
 	 *
+	 * @since n.e.x.t
+	 *
 	 * @return string XPath.
 	 */
 	public function get_xpath(): string {
@@ -266,6 +281,7 @@ public function get_xpath(): string {
 	 * This is a wrapper around the underlying HTML_Tag_Processor method of the same name since only a limited number of
 	 * methods can be exposed to prevent moving the pointer in such a way as the breadcrumb calculation is invalidated.
 	 *
+	 * @since n.e.x.t
 	 * @see WP_HTML_Tag_Processor::get_attribute()
 	 *
 	 * @param string $name Name of attribute whose value is requested.
@@ -281,6 +297,7 @@ public function get_attribute( string $name ) {
 	 * This is a wrapper around the underlying HTML_Tag_Processor method of the same name since only a limited number of
 	 * methods can be exposed to prevent moving the pointer in such a way as the breadcrumb calculation is invalidated.
 	 *
+	 * @since n.e.x.t
 	 * @see WP_HTML_Tag_Processor::set_attribute()
 	 *
 	 * @param string      $name  The attribute name to target.
@@ -297,6 +314,7 @@ public function set_attribute( string $name, $value ): bool {
 	 * This is a wrapper around the underlying HTML_Tag_Processor method of the same name since only a limited number of
 	 * methods can be exposed to prevent moving the pointer in such a way as the breadcrumb calculation is invalidated.
 	 *
+	 * @since n.e.x.t
 	 * @see WP_HTML_Tag_Processor::remove_attribute()
 	 *
 	 * @param string $name The attribute name to remove.
@@ -312,6 +330,7 @@ public function remove_attribute( string $name ): bool {
 	 * This is a wrapper around the underlying HTML_Tag_Processor method of the same name since only a limited number of
 	 * methods can be exposed to prevent moving the pointer in such a way as the breadcrumb calculation is invalidated.
 	 *
+	 * @since n.e.x.t
 	 * @see WP_HTML_Tag_Processor::get_updated_html()
 	 *
 	 * @return string The processed HTML.
diff --git a/modules/images/image-loading-optimization/detection.php b/modules/images/image-loading-optimization/detection.php
index 288d45714..7eb999ec6 100644
--- a/modules/images/image-loading-optimization/detection.php
+++ b/modules/images/image-loading-optimization/detection.php
@@ -33,7 +33,7 @@ function ilo_get_detection_script( string $slug, array $needed_minimum_viewport_
 	 *
 	 * @param int $detection_time_window Detection time window in milliseconds.
 	 */
-	$detection_time_window = apply_filters( 'perflab_image_loading_detection_time_window', 5000 );
+	$detection_time_window = apply_filters( 'ilo_detection_time_window', 5000 );
 
 	$detect_args = array(
 		'serveTime'                   => microtime( true ) * 1000, // In milliseconds for comparison with `Date.now()` in JavaScript.
diff --git a/modules/images/image-loading-optimization/optimization.php b/modules/images/image-loading-optimization/optimization.php
index 6e5a2c13d..1408014c0 100644
--- a/modules/images/image-loading-optimization/optimization.php
+++ b/modules/images/image-loading-optimization/optimization.php
@@ -24,32 +24,65 @@ function ilo_maybe_add_template_output_buffer_filter() {
 }
 add_action( 'wp', 'ilo_maybe_add_template_output_buffer_filter' );
 
+/**
+ * Determines whether the current response can be optimized.
+ *
+ * Only search results are not eligible by default for optimization. This is because there is no predictability in
+ * whether posts in the loop will have featured images assigned or not. If a theme template for search results doesn't
+ * even show featured images, then this isn't an issue.
+ *
+ * @since n.e.x.t
+ * @access private
+ *
+ * @return bool Whether response can be optimized.
+ */
+function ilo_can_optimize_response(): bool {
+	$able = ! (
+		// Since the URL space is infinite.
+		is_search() ||
+		// Since injection of inline-editing controls interfere with breadcrumbs, while also just not necessary in this context.
+		is_customize_preview() ||
+		// The images detected in the response body of a POST request cannot, by definition, be cached.
+		'GET' !== $_SERVER['REQUEST_METHOD']
+	);
+
+	/**
+	 * Filters whether the current response can be optimized.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param bool $able Whether response can be optimized.
+	 */
+	return (bool) apply_filters( 'ilo_can_optimize_response', $able );
+}
+
 /**
  * Constructs preload links.
  *
  * @since n.e.x.t
  * @access private
  *
- * @param array $lcp_images_by_minimum_viewport_widths LCP images keyed by minimum viewport width, amended with attributes key for the IMG attributes.
+ * @param array<int, array{attributes: array{src?: string, srcset?: string, sizes?: string, crossorigin?: string}}|false> $lcp_elements_by_minimum_viewport_widths LCP images keyed by minimum viewport width, amended with attributes key for the IMG attributes.
  * @return string Markup for zero or more preload link tags.
  */
-function ilo_construct_preload_links( array $lcp_images_by_minimum_viewport_widths ): string {
+function ilo_construct_preload_links( array $lcp_elements_by_minimum_viewport_widths ): string {
 	$preload_links = array();
 
 	// This uses a for loop to be able to access the following element within the iteration, using a numeric index.
-	$minimum_viewport_widths = array_keys( $lcp_images_by_minimum_viewport_widths );
+	$minimum_viewport_widths = array_keys( $lcp_elements_by_minimum_viewport_widths );
 	for ( $i = 0, $len = count( $minimum_viewport_widths ); $i < $len; $i++ ) {
-		$lcp_element = $lcp_images_by_minimum_viewport_widths[ $minimum_viewport_widths[ $i ] ];
-		if ( false === $lcp_element || empty( $lcp_element['attributes'] ) ) {
-			// No LCP element at this breakpoint, so nothing to preload.
+		$lcp_element = $lcp_elements_by_minimum_viewport_widths[ $minimum_viewport_widths[ $i ] ];
+		if ( false === $lcp_element ) {
+			// No supported LCP element at this breakpoint, so nothing to preload.
 			continue;
 		}
 
-		$img_attributes = $lcp_element['attributes'];
+		// TODO: Add support for background images.
+		$attributes = $lcp_element['attributes'];
 
 		// Prevent preloading src for browsers that don't support imagesrcset on the link element.
-		if ( isset( $img_attributes['src'], $img_attributes['srcset'] ) ) {
-			unset( $img_attributes['src'] );
+		if ( isset( $attributes['src'], $attributes['srcset'] ) ) {
+			unset( $attributes['src'] );
 		}
 
 		// Add media query if it's going to be something other than just `min-width: 0px`.
@@ -60,12 +93,12 @@ function ilo_construct_preload_links( array $lcp_images_by_minimum_viewport_widt
 			if ( null !== $maximum_viewport_width ) {
 				$media_query .= sprintf( ' and ( max-width: %dpx )', $maximum_viewport_width );
 			}
-			$img_attributes['media'] = $media_query;
+			$attributes['media'] = $media_query;
 		}
 
 		// Construct preload link.
 		$link_tag = '<link data-ilo-added-tag rel="preload" fetchpriority="high" as="image"';
-		foreach ( array_filter( $img_attributes ) as $name => $value ) {
+		foreach ( array_filter( $attributes ) as $name => $value ) {
 			// Map img attribute name to link attribute name.
 			if ( 'srcset' === $name || 'sizes' === $name ) {
 				$name = 'image' . $name;
@@ -107,7 +140,12 @@ function ilo_optimize_template_output_buffer( string $buffer ): string {
 	);
 
 	// Whether we need to add the data-ilo-xpath attribute to elements and whether the detection script should be injected.
-	$needs_detection = ilo_needs_url_metric_for_breakpoint( $needed_minimum_viewport_widths );
+	$needs_detection = in_array(
+		true,
+		// Each array item is array{int, bool}, with the second item being whether the viewport width is needed.
+		array_column( $needed_minimum_viewport_widths, 1 ),
+		true
+	);
 
 	$breakpoint_max_widths                   = ilo_get_breakpoint_max_widths();
 	$url_metrics_grouped_by_breakpoint       = ilo_group_url_metrics_by_breakpoint( $url_metrics, $breakpoint_max_widths );
@@ -169,10 +207,13 @@ function ilo_optimize_template_output_buffer( string $buffer ): string {
 			$processor->remove_attribute( 'fetchpriority' );
 		}
 
+		// TODO: If the image is visible (intersectionRatio!=0) in any of the URL metrics, remove loading=lazy.
+		// TODO: Conversely, if an image is the LCP element for one breakpoint but not another, add loading=lazy. This won't hurt performance since the image is being preloaded.
+
 		// Capture the attributes from the LCP elements to use in preload links.
 		if ( isset( $lcp_element_minimum_viewport_width_by_xpath[ $xpath ] ) ) {
 			$attributes = array();
-			foreach ( array( 'src', 'srcset', 'sizes', 'crossorigin', 'integrity' ) as $attr_name ) {
+			foreach ( array( 'src', 'srcset', 'sizes', 'crossorigin' ) as $attr_name ) {
 				$attributes[ $attr_name ] = $processor->get_attribute( $attr_name );
 			}
 			foreach ( $lcp_element_minimum_viewport_width_by_xpath[ $xpath ] as $minimum_viewport_width ) {
diff --git a/modules/images/image-loading-optimization/storage/data.php b/modules/images/image-loading-optimization/storage/data.php
index 35eda471b..09840b205 100644
--- a/modules/images/image-loading-optimization/storage/data.php
+++ b/modules/images/image-loading-optimization/storage/data.php
@@ -31,38 +31,6 @@ function ilo_get_url_metric_freshness_ttl(): int {
 	return (int) apply_filters( 'ilo_url_metric_freshness_ttl', DAY_IN_SECONDS );
 }
 
-/**
- * Determines whether the current response can be optimized.
- *
- * Only search results are not eligible by default for optimization. This is because there is no predictability in
- * whether posts in the loop will have featured images assigned or not. If a theme template for search results doesn't
- * even show featured images, then this isn't an issue.
- *
- * @since n.e.x.t
- * @access private
- *
- * @return bool Whether response can be optimized.
- */
-function ilo_can_optimize_response(): bool {
-	$able = ! (
-		// Since the URL space is infinite.
-		is_search() ||
-		// Since injection of inline-editing controls interfere with breadcrumbs, while also just not necessary in this context.
-		is_customize_preview() ||
-		// The images detected in the response body of a POST request cannot, by definition, be cached.
-		'GET' !== $_SERVER['REQUEST_METHOD']
-	);
-
-	/**
-	 * Filters whether the current response can be optimized.
-	 *
-	 * @since n.e.x.t
-	 *
-	 * @param bool $able Whether response can be optimized.
-	 */
-	return (bool) apply_filters( 'ilo_can_optimize_response', $able );
-}
-
 /**
  * Gets the normalized query vars for the current request.
  *
@@ -136,12 +104,10 @@ function ilo_get_url_metrics_storage_nonce( string $slug ): string {
  *
  * @param string $nonce URL metrics storage nonce.
  * @param string $slug  URL metrics slug.
- * @return int 1 if the nonce is valid and generated between 0-12 hours ago,
- *             2 if the nonce is valid and generated between 12-24 hours ago.
- *             0 if the nonce is invalid.
+ * @return bool Whether the nonce is valid.
  */
-function ilo_verify_url_metrics_storage_nonce( string $nonce, string $slug ): int {
-	return (int) wp_verify_nonce( $nonce, "store_url_metrics:$slug" );
+function ilo_verify_url_metrics_storage_nonce( string $nonce, string $slug ): bool {
+	return (bool) wp_verify_nonce( $nonce, "store_url_metrics:$slug" );
 }
 
 /**
@@ -150,33 +116,40 @@ function ilo_verify_url_metrics_storage_nonce( string $nonce, string $slug ): in
  * @since n.e.x.t
  * @access private
  *
- * @param array $url_metrics          URL metrics.
+ * @param array $url_metrics          URL metrics. Each URL metric is expected to have a timestamp key.
  * @param array $validated_url_metric Validated URL metric. See JSON Schema defined in ilo_register_endpoint().
- * @return array Updated URL metrics.
+ * @param int[] $breakpoints          Breakpoint max widths.
+ * @param int   $sample_size          Sample size for URL metrics at a given breakpoint.
+ * @return array Updated URL metrics, with timestamp key added.
  */
-function ilo_unshift_url_metrics( array $url_metrics, array $validated_url_metric ): array {
+function ilo_unshift_url_metrics( array $url_metrics, array $validated_url_metric, array $breakpoints, int $sample_size ): array {
+	$validated_url_metric['timestamp'] = microtime( true );
 	array_unshift( $url_metrics, $validated_url_metric );
-	$breakpoints         = ilo_get_breakpoint_max_widths();
-	$sample_size         = ilo_get_url_metrics_breakpoint_sample_size();
 	$grouped_url_metrics = ilo_group_url_metrics_by_breakpoint( $url_metrics, $breakpoints );
 
-	foreach ( $grouped_url_metrics as &$breakpoint_url_metrics ) {
-		if ( count( $breakpoint_url_metrics ) > $sample_size ) {
-
-			// Sort URL metrics in descending order by timestamp.
-			usort(
-				$breakpoint_url_metrics,
-				static function ( $a, $b ) {
-					if ( ! isset( $a['timestamp'] ) || ! isset( $b['timestamp'] ) ) {
-						return 0;
+	// Make sure there is at most $sample_size number of URL metrics for each breakpoint.
+	$grouped_url_metrics = array_map(
+		static function ( $breakpoint_url_metrics ) use ( $sample_size ) {
+			if ( count( $breakpoint_url_metrics ) > $sample_size ) {
+
+				// Sort URL metrics in descending order by timestamp.
+				usort(
+					$breakpoint_url_metrics,
+					static function ( $a, $b ) {
+						if ( ! isset( $a['timestamp'] ) || ! isset( $b['timestamp'] ) ) {
+							return 0;
+						}
+						return $b['timestamp'] <=> $a['timestamp'];
 					}
-					return $b['timestamp'] <=> $a['timestamp'];
-				}
-			);
+				);
 
-			$breakpoint_url_metrics = array_slice( $breakpoint_url_metrics, 0, $sample_size );
-		}
-	}
+				// Only keep the sample size of the newest URL metrics.
+				$breakpoint_url_metrics = array_slice( $breakpoint_url_metrics, 0, $sample_size );
+			}
+			return $breakpoint_url_metrics;
+		},
+		$grouped_url_metrics
+	);
 
 	return array_merge( ...$grouped_url_metrics );
 }
@@ -266,24 +239,23 @@ function ilo_get_url_metrics_breakpoint_sample_size(): int {
 function ilo_group_url_metrics_by_breakpoint( array $url_metrics, array $breakpoints ): array {
 
 	// Convert breakpoint max widths into viewport minimum widths.
-	$viewport_minimum_widths = array_map(
+	$minimum_viewport_widths = array_map(
 		static function ( $breakpoint ) {
 			return $breakpoint + 1;
 		},
 		$breakpoints
 	);
 
-	$grouped = array_fill_keys( array_merge( array( 0 ), $viewport_minimum_widths ), array() );
+	$grouped = array_fill_keys( array_merge( array( 0 ), $minimum_viewport_widths ), array() );
 
 	foreach ( $url_metrics as $url_metric ) {
 		if ( ! isset( $url_metric['viewport']['width'] ) ) {
 			continue;
 		}
-		$viewport_width = $url_metric['viewport']['width'];
 
 		$current_minimum_viewport = 0;
-		foreach ( $viewport_minimum_widths as $viewport_minimum_width ) {
-			if ( $viewport_width > $viewport_minimum_width ) {
+		foreach ( $minimum_viewport_widths as $viewport_minimum_width ) {
+			if ( $url_metric['viewport']['width'] > $viewport_minimum_width ) {
 				$current_minimum_viewport = $viewport_minimum_width;
 			} else {
 				break;
@@ -299,15 +271,16 @@ static function ( $breakpoint ) {
  * Gets the LCP element for each breakpoint.
  *
  * The array keys are the minimum viewport width required for the element to be LCP. If there are URL metrics for a
- * given breakpoint and yet there is no LCP element, then the array value is `false`. If there is an LCP element at the
- * breakpoint, then the array value is an array representing that element, including its breadcrumbs. If two adjoining
- * breakpoints have the same value, then the latter is dropped.
+ * given breakpoint and yet there is no supported LCP element, then the array value is `false`. (Currently only IMG is
+ * a supported LCP element.) If there is a supported LCP element at the breakpoint, then the array value is an array
+ * representing that element, including its breadcrumbs. If two adjoining breakpoints have the same value, then the
+ * latter is dropped.
  *
  * @since n.e.x.t
  * @access private
  *
  * @param array $grouped_url_metrics URL metrics grouped by breakpoint. See `ilo_group_url_metrics_by_breakpoint()`.
- * @return array LCP elements keyed by its minimum viewport width. If there is no LCP element at a breakpoint, then `false` is used.
+ * @return array LCP elements keyed by its minimum viewport width. If there is no supported LCP element at a breakpoint, then `false` is used.
  */
 function ilo_get_lcp_elements_by_minimum_viewport_widths( array $grouped_url_metrics ): array {
 
@@ -382,8 +355,8 @@ static function ( $lcp_element ) use ( &$prev_lcp_element ) {
  * @since n.e.x.t
  * @access private
  *
- * @param array $url_metrics           URL metrics.
- * @param float $current_time           Current time as returned by microtime(true).
+ * @param array $url_metrics            URL metrics.
+ * @param float $current_time           Current time as returned by `microtime(true)`.
  * @param int[] $breakpoint_max_widths  Breakpoint max widths.
  * @param int   $sample_size            Sample size for viewports in a breakpoint.
  * @param int   $freshness_ttl          Freshness TTL for a URL metric.
@@ -412,21 +385,3 @@ function ilo_get_needed_minimum_viewport_widths( array $url_metrics, float $curr
 
 	return $needed_minimum_viewport_widths;
 }
-
-/**
- * Checks whether there is a URL metric needed for one of the breakpoints.
- *
- * @since n.e.x.t
- * @access private
- *
- * @param array<int, array{int, bool}> $needed_minimum_viewport_widths Array of tuples mapping minimum viewport width to whether URL metric(s) are needed.
- * @return bool Whether a URL metric is needed.
- */
-function ilo_needs_url_metric_for_breakpoint( array $needed_minimum_viewport_widths ): bool {
-	foreach ( $needed_minimum_viewport_widths as list( $minimum_viewport_width, $is_needed ) ) {
-		if ( $is_needed ) {
-			return true;
-		}
-	}
-	return false;
-}
diff --git a/modules/images/image-loading-optimization/storage/post-type.php b/modules/images/image-loading-optimization/storage/post-type.php
index a95a71176..00d397af0 100644
--- a/modules/images/image-loading-optimization/storage/post-type.php
+++ b/modules/images/image-loading-optimization/storage/post-type.php
@@ -110,7 +110,40 @@ function ilo_parse_stored_url_metrics( WP_Post $post ): array {
 		);
 		$url_metrics = array();
 	}
-	return $url_metrics;
+
+	return array_values(
+		array_filter(
+			$url_metrics,
+			static function ( $url_metric ) use ( $trigger_error ) {
+				// TODO: If we wanted, we could use the JSON Schema to validate the stored metrics.
+				$is_valid = (
+					is_array( $url_metric )
+					&&
+					isset(
+						$url_metric['viewport']['width'],
+						$url_metric['viewport']['height'],
+						$url_metric['elements']
+					)
+					&&
+					is_int( $url_metric['viewport']['width'] )
+					&&
+					is_array( $url_metric['elements'] )
+				);
+
+				if ( ! $is_valid ) {
+					$trigger_error(
+						sprintf(
+							/* translators: %s is post type slug */
+							__( 'Unexpected shape to JSON array in post_content of %s post type.', 'performance-lab' ),
+							ILO_URL_METRICS_POST_TYPE
+						)
+					);
+				}
+
+				return $is_valid;
+			}
+		)
+	);
 }
 
 /**
@@ -119,13 +152,12 @@ function ilo_parse_stored_url_metrics( WP_Post $post ): array {
  * @since n.e.x.t
  * @access private
  *
- * @param string $url                   URL for the URL metrics. This is used purely as metadata.
- * @param string $slug                  URL metrics slug (computed from query vars).
+ * @param string $url                  URL for the URL metrics. This is used purely as metadata.
+ * @param string $slug                 URL metrics slug (computed from query vars).
  * @param array  $validated_url_metric Validated URL metric. See JSON Schema defined in ilo_register_endpoint().
  * @return int|WP_Error Post ID or WP_Error otherwise.
  */
 function ilo_store_url_metric( string $url, string $slug, array $validated_url_metric ) {
-	$validated_url_metric['timestamp'] = microtime( true );
 
 	// TODO: What about storing a version identifier?
 	$post_data = array(
@@ -143,7 +175,9 @@ function ilo_store_url_metric( string $url, string $slug, array $validated_url_m
 		$url_metrics            = array();
 	}
 
-	$url_metrics = ilo_unshift_url_metrics( $url_metrics, $validated_url_metric );
+	$breakpoints = ilo_get_breakpoint_max_widths();
+	$sample_size = ilo_get_url_metrics_breakpoint_sample_size();
+	$url_metrics = ilo_unshift_url_metrics( $url_metrics, $validated_url_metric, $breakpoints, $sample_size );
 
 	$post_data['post_content'] = wp_json_encode( $url_metrics, JSON_PRETTY_PRINT | JSON_UNESCAPED_SLASHES ); // TODO: No need for pretty-printing.
 
diff --git a/modules/images/image-loading-optimization/storage/rest-api.php b/modules/images/image-loading-optimization/storage/rest-api.php
index f12f3c36c..7ce1b06c9 100644
--- a/modules/images/image-loading-optimization/storage/rest-api.php
+++ b/modules/images/image-loading-optimization/storage/rest-api.php
@@ -105,12 +105,12 @@ function ilo_register_endpoint() {
 					'required'    => true,
 					'properties'  => array(
 						'width'  => array(
-							'type'     => 'int',
+							'type'     => 'integer',
 							'required' => true,
 							'minimum'  => 0,
 						),
 						'height' => array(
-							'type'     => 'int',
+							'type'     => 'integer',
 							'required' => true,
 							'minimum'  => 0,
 						),
@@ -119,16 +119,17 @@ function ilo_register_endpoint() {
 				'elements' => array(
 					'description' => __( 'Element metrics', 'performance-lab' ),
 					'type'        => 'array',
+					'required'    => true,
 					'items'       => array(
 						// See the ElementMetrics in detect.js.
 						'type'       => 'object',
 						'properties' => array(
 							'isLCP'              => array(
-								'type'     => 'bool',
+								'type'     => 'boolean',
 								'required' => true,
 							),
 							'isLCPCandidate'     => array(
-								'type' => 'bool',
+								'type' => 'boolean',
 							),
 							'xpath'              => array(
 								'type'     => 'string',
@@ -171,16 +172,28 @@ function ilo_handle_rest_request( WP_REST_Request $request ) {
 		ilo_get_url_metrics_breakpoint_sample_size(),
 		ilo_get_url_metric_freshness_ttl()
 	);
-	if ( ! ilo_needs_url_metric_for_breakpoint( $needed_minimum_viewport_widths ) ) {
+
+	// Block the request if URL metrics aren't needed for the provided viewport width.
+	// This logic is the same as the isViewportNeeded() function in detect.js.
+	$viewport_width  = $request->get_param( 'viewport' )['width'];
+	$last_was_needed = false;
+	foreach ( $needed_minimum_viewport_widths as list( $minimum_viewport_width, $is_needed ) ) {
+		if ( $viewport_width >= $minimum_viewport_width ) {
+			$last_was_needed = $is_needed;
+		} else {
+			break;
+		}
+	}
+	if ( ! $last_was_needed ) {
 		return new WP_Error(
 			'no_url_metric_needed',
-			__( 'No URL metric needed for any of the breakpoints.', 'performance-lab' ),
+			__( 'No URL metric needed for the provided viewport width.', 'performance-lab' ),
 			array( 'status' => 403 )
 		);
 	}
 
 	ilo_set_url_metric_storage_lock();
-	$new_url_metric = wp_array_slice_assoc( $request->get_json_params(), array( 'viewport', 'elements' ) );
+	$new_url_metric = wp_array_slice_assoc( $request->get_params(), array( 'viewport', 'elements' ) );
 
 	$result = ilo_store_url_metric(
 		$request->get_param( 'url' ),
@@ -195,8 +208,6 @@ function ilo_handle_rest_request( WP_REST_Request $request ) {
 	return new WP_REST_Response(
 		array(
 			'success' => true,
-			'post_id' => $result,
-			'data'    => ilo_parse_stored_url_metrics( ilo_get_url_metrics_post( $request->get_param( 'slug' ) ) ), // TODO: Remove this debug data.
 		)
 	);
 }
diff --git a/tests/admin/load-tests.php b/tests/admin/load-tests.php
index c7746f9bf..83e4555d8 100644
--- a/tests/admin/load-tests.php
+++ b/tests/admin/load-tests.php
@@ -91,7 +91,7 @@ public function test_perflab_add_modules_page() {
 		remove_all_filters( 'plugin_action_links_' . plugin_basename( PERFLAB_MAIN_FILE ) );
 
 		// Does not register the page if the perflab_active_modules filter is used.
-		add_filter( 'perflab_active_modules', '__return_null' );
+		add_filter( 'perflab_active_modules', '__return_array' );
 		$hook_suffix = perflab_add_modules_page();
 		$this->assertFalse( $hook_suffix );
 		$this->assertFalse( isset( $_wp_submenu_nopriv['options-general.php'][ PERFLAB_MODULES_SCREEN ] ) );
diff --git a/tests/modules/images/image-loading-optimization/class-ilo-html-tag-processor-tests.php b/tests/modules/images/image-loading-optimization/class-ilo-html-tag-processor-tests.php
new file mode 100644
index 000000000..6cd0c7bbb
--- /dev/null
+++ b/tests/modules/images/image-loading-optimization/class-ilo-html-tag-processor-tests.php
@@ -0,0 +1,333 @@
+<?php
+/**
+ * Tests for image-loading-optimization class ILO_HTML_Tag_Processor.
+ *
+ * @package performance-lab
+ * @group   image-loading-optimization
+ *
+ * @coversDefaultClass ILO_HTML_Tag_Processor
+ */
+class ILO_HTML_Tag_Processor_Tests extends WP_UnitTestCase {
+
+	public function data_provider_sample_documents(): array {
+		return array(
+			'well-formed-html'   => array(
+				'document'  => '
+					<!DOCTYPE html>
+					<html>
+						<head>
+							<meta charset="utf8">
+							<title>Foo</title>
+						</head>
+						<body>
+							<p>
+								Foo!
+								<br>
+								<img src="/foo.jpg" width="1000" height="600" alt="Foo">
+							</p>
+							<footer>The end!</footer>
+						</body>
+					</html>
+				',
+				'open_tags' => array( 'HTML', 'HEAD', 'META', 'TITLE', 'BODY', 'P', 'BR', 'IMG', 'FOOTER' ),
+				'xpaths'    => array(
+					'/*[0][self::HTML]',
+					'/*[0][self::HTML]/*[0][self::HEAD]',
+					'/*[0][self::HTML]/*[0][self::HEAD]/*[0][self::META]',
+					'/*[0][self::HTML]/*[0][self::HEAD]/*[1][self::TITLE]',
+					'/*[0][self::HTML]/*[1][self::BODY]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::P]/*[0][self::BR]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::P]/*[1][self::IMG]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::FOOTER]',
+				),
+			),
+			'foreign-elements'   => array(
+				'document'  => '
+					<html>
+						<head></head>
+						<body>
+							<svg>
+								<g>
+									<path d="M10 10"/>
+									<circle cx="10" cy="10" r="2" fill="red"/>
+									<g />
+									<rect width="100%" height="100%" fill="red" />
+								</g>
+							</svg>
+							<math display="block">
+								<mn>1</mn>
+								<mspace depth="40px" height="20px" width="100px" style="background: lightblue;"/>
+								<mn>2</mn>
+							</math>
+						</body>
+					</html>
+				',
+				'open_tags' => array( 'HTML', 'HEAD', 'BODY', 'SVG', 'G', 'PATH', 'CIRCLE', 'G', 'RECT', 'MATH', 'MN', 'MSPACE', 'MN' ),
+				'xpaths'    => array(
+					'/*[0][self::HTML]',
+					'/*[0][self::HTML]/*[0][self::HEAD]',
+					'/*[0][self::HTML]/*[1][self::BODY]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::SVG]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::SVG]/*[0][self::G]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::SVG]/*[0][self::G]/*[0][self::PATH]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::SVG]/*[0][self::G]/*[1][self::CIRCLE]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::SVG]/*[0][self::G]/*[2][self::G]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::SVG]/*[0][self::G]/*[3][self::RECT]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::MATH]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::MATH]/*[0][self::MN]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::MATH]/*[1][self::MSPACE]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::MATH]/*[2][self::MN]',
+				),
+			),
+			'closing-void-tag'   => array(
+				'document'  => '
+					<html>
+						<head></head>
+						<body>
+							<span>1</span>
+							<br></br>
+							<span>2</span>
+						</body>
+					</html>
+				',
+				'open_tags' => array( 'HTML', 'HEAD', 'BODY', 'SPAN', 'BR', 'SPAN' ),
+				'xpaths'    => array(
+					'/*[0][self::HTML]',
+					'/*[0][self::HTML]/*[0][self::HEAD]',
+					'/*[0][self::HTML]/*[1][self::BODY]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::SPAN]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::BR]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[2][self::SPAN]',
+				),
+			),
+			'void-tags'          => array(
+				'document'  => '
+					<html>
+						<head></head>
+						<body>
+							<area>
+							<base>
+							<basefont>
+							<bgsound>
+							<br>
+							<col>
+							<embed>
+							<frame>
+							<hr>
+							<img>
+							<input>
+							<keygen>
+							<link>
+							<meta>
+							<param>
+							<source>
+							<track>
+							<wbr>
+
+							<!-- The following are not void -->
+							<div>
+							<span>
+							<em>
+						</body>
+					</html>
+				',
+				'open_tags' => array( 'HTML', 'HEAD', 'BODY', 'AREA', 'BASE', 'BASEFONT', 'BGSOUND', 'BR', 'COL', 'EMBED', 'FRAME', 'HR', 'IMG', 'INPUT', 'KEYGEN', 'LINK', 'META', 'PARAM', 'SOURCE', 'TRACK', 'WBR', 'DIV', 'SPAN', 'EM' ),
+				'xpaths'    => array(
+					'/*[0][self::HTML]',
+					'/*[0][self::HTML]/*[0][self::HEAD]',
+					'/*[0][self::HTML]/*[1][self::BODY]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::AREA]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::BASE]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[2][self::BASEFONT]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[3][self::BGSOUND]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[4][self::BR]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[5][self::COL]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[6][self::EMBED]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[7][self::FRAME]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[8][self::HR]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[9][self::IMG]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[10][self::INPUT]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[11][self::KEYGEN]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[12][self::LINK]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[13][self::META]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[14][self::PARAM]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[15][self::SOURCE]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[16][self::TRACK]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[17][self::WBR]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[18][self::DIV]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[18][self::DIV]/*[0][self::SPAN]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[18][self::DIV]/*[0][self::SPAN]/*[0][self::EM]',
+				),
+			),
+			'optional-closing-p' => array(
+				'document'  => '
+					<html>
+						<head></head>
+						<body>
+							<!-- In HTML, the closing paragraph tag is optional. -->
+							<p>First
+							<p><em>Second</em>
+							<p>Third
+
+							<!-- Try triggering all closing -->
+							<p><address></address>
+							<p><article></article>
+							<p><aside></aside>
+							<p><blockquote></blockquote>
+							<p><details></details>
+							<p><div></div>
+							<p><dl></dl>
+							<p><fieldset></fieldset>
+							<p><figcaption></figcaption>
+							<p><figure></figure>
+							<p><footer></footer>
+							<p><form></form>
+							<p><h1></h1>
+							<p><h2></h2>
+							<p><h3></h3>
+							<p><h4></h4>
+							<p><h5></h5>
+							<p><h6></h6>
+							<p><header></header>
+							<p><hgroup></hgroup>
+							<p><hr>
+							<p><main></main>
+							<p><menu></menu>
+							<p><nav></nav>
+							<p><ol></ol>
+							<p><pre></pre>
+							<p><search></search>
+							<p><section></section>
+							<p><table></table>
+							<p><ul></ul>
+						</body>
+					</html>
+				',
+				'open_tags' => array( 'HTML', 'HEAD', 'BODY', 'P', 'P', 'EM', 'P', 'P', 'ADDRESS', 'P', 'ARTICLE', 'P', 'ASIDE', 'P', 'BLOCKQUOTE', 'P', 'DETAILS', 'P', 'DIV', 'P', 'DL', 'P', 'FIELDSET', 'P', 'FIGCAPTION', 'P', 'FIGURE', 'P', 'FOOTER', 'P', 'FORM', 'P', 'H1', 'P', 'H2', 'P', 'H3', 'P', 'H4', 'P', 'H5', 'P', 'H6', 'P', 'HEADER', 'P', 'HGROUP', 'P', 'HR', 'P', 'MAIN', 'P', 'MENU', 'P', 'NAV', 'P', 'OL', 'P', 'PRE', 'P', 'SEARCH', 'P', 'SECTION', 'P', 'TABLE', 'P', 'UL' ),
+				'xpaths'    => array(
+					'/*[0][self::HTML]',
+					'/*[0][self::HTML]/*[0][self::HEAD]',
+					'/*[0][self::HTML]/*[1][self::BODY]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::P]/*[0][self::EM]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::ADDRESS]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::ARTICLE]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::ASIDE]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::BLOCKQUOTE]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::DETAILS]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::DIV]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::DL]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::FIELDSET]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::FIGCAPTION]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::FIGURE]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::FOOTER]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::FORM]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::H1]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::H2]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::H3]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::H4]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::H5]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::H6]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::HEADER]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::HGROUP]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::HR]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::MAIN]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::MENU]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::NAV]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::OL]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::PRE]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::SEARCH]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::SECTION]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::TABLE]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[1][self::P]',
+					'/*[0][self::HTML]/*[1][self::BODY]/*[0][self::UL]',
+				),
+			),
+		);
+	}
+
+	/**
+	 * Test open_tags() and get_xpath().
+	 *
+	 * @covers ::open_tags
+	 * @covers ::get_xpath
+	 *
+	 * @dataProvider data_provider_sample_documents
+	 */
+	public function test_open_tags_and_get_xpath( string $document, array $open_tags, array $xpaths ) {
+		$p = new ILO_HTML_Tag_Processor( $document );
+		$this->assertSame( '', $p->get_xpath(), 'Expected empty XPath since iteration has not started.' );
+		$actual_open_tags = array();
+		$actual_xpaths    = array();
+		foreach ( $p->open_tags() as $open_tag ) {
+			$actual_open_tags[] = $open_tag;
+			$actual_xpaths[]    = $p->get_xpath();
+		}
+
+		$this->assertSame( $actual_open_tags, $open_tags, "Expected list of open tags to match.\nSnapshot: " . $this->export_array_snapshot( $actual_open_tags, true ) );
+		$this->assertSame( $actual_xpaths, $xpaths, "Expected list of XPaths to match.\nSnapshot:" . $this->export_array_snapshot( $actual_xpaths ) );
+	}
+
+	/**
+	 * Test get_attribute(), set_attribute(), remove_attribute(), and get_updated_html().
+	 *
+	 * @covers ::get_attribute
+	 * @covers ::set_attribute
+	 * @covers ::remove_attribute
+	 * @covers ::get_updated_html
+	 */
+	public function test_html_tag_processor_wrapper_methods() {
+		$processor = new ILO_HTML_Tag_Processor( '<html lang="en" xml:lang="en"></html>' );
+		foreach ( $processor->open_tags() as $open_tag ) {
+			if ( 'HTML' === $open_tag ) {
+				$this->assertSame( 'en', $processor->get_attribute( 'lang' ) );
+				$processor->set_attribute( 'lang', 'es' );
+				$processor->remove_attribute( 'xml:lang' );
+			}
+		}
+		$this->assertSame( '<html lang="es" ></html>', $processor->get_updated_html() );
+	}
+
+	/**
+	 * Export an array as a PHP literal to use as a snapshot.
+	 */
+	private function export_array_snapshot( array $data, bool $one_line = false ): string {
+		$php = preg_replace( '/^\s*\d+\s*=>\s*/m', '', var_export( $data, true ) );
+		if ( $one_line ) {
+			$php = str_replace( "\n", ' ', $php );
+		}
+		return $php;
+	}
+}
diff --git a/tests/modules/images/image-loading-optimization/detection-tests.php b/tests/modules/images/image-loading-optimization/detection-tests.php
new file mode 100644
index 000000000..712f31aec
--- /dev/null
+++ b/tests/modules/images/image-loading-optimization/detection-tests.php
@@ -0,0 +1,74 @@
+<?php
+/**
+ * Tests for image-loading-optimization module detection.php.
+ *
+ * @package performance-lab
+ * @group   image-loading-optimization
+ */
+
+class ILO_Detection_Tests extends WP_UnitTestCase {
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array<string, array{set_up: Closure, expected_exports: array<string, mixed>}>
+	 */
+	public function data_provider_ilo_get_detection_script(): array {
+		return array(
+			'unfiltered' => array(
+				'set_up'           => static function () {},
+				'expected_exports' => array(
+					'detectionTimeWindow' => 5000,
+					'storageLockTTL'      => MINUTE_IN_SECONDS,
+				),
+			),
+			'filtered'   => array(
+				'set_up'           => static function () {
+					add_filter(
+						'ilo_detection_time_window',
+						static function (): int {
+							return 2500;
+						}
+					);
+					add_filter(
+						'ilo_url_metric_storage_lock_ttl',
+						static function (): int {
+							return HOUR_IN_SECONDS;
+						}
+					);
+				},
+				'expected_exports' => array(
+					'detectionTimeWindow' => 2500,
+					'storageLockTTL'      => HOUR_IN_SECONDS,
+				),
+			),
+		);
+	}
+
+	/**
+	 * Make sure the expected script is printed.
+	 *
+	 * @covers ::ilo_get_detection_script
+	 *
+	 * @dataProvider data_provider_ilo_get_detection_script
+	 *
+	 * @param Closure                                                                       $set_up           Set up callback.
+	 * @param array<string, array{set_up: Closure, expected_exports: array<string, mixed>}> $expected_exports Expected exports.
+	 */
+	public function test_ilo_get_detection_script_returns_script( Closure $set_up, array $expected_exports ) {
+		$set_up();
+		$slug                           = ilo_get_url_metrics_slug( array( 'p' => '1' ) );
+		$needed_minimum_viewport_widths = array(
+			array( 480, false ),
+			array( 600, false ),
+			array( 782, true ),
+		);
+		$script                         = ilo_get_detection_script( $slug, $needed_minimum_viewport_widths );
+
+		$this->assertStringContainsString( '<script type="module">', $script );
+		$this->assertStringContainsString( 'import detect from', $script );
+		foreach ( $expected_exports as $key => $value ) {
+			$this->assertStringContainsString( sprintf( '%s:%s', wp_json_encode( $key ), wp_json_encode( $value ) ), $script );
+		}
+	}
+}
diff --git a/tests/modules/images/image-loading-optimization/load-tests.php b/tests/modules/images/image-loading-optimization/hooks-tests.php
similarity index 77%
rename from tests/modules/images/image-loading-optimization/load-tests.php
rename to tests/modules/images/image-loading-optimization/hooks-tests.php
index 661d876f8..8ca9cdddd 100644
--- a/tests/modules/images/image-loading-optimization/load-tests.php
+++ b/tests/modules/images/image-loading-optimization/hooks-tests.php
@@ -1,31 +1,26 @@
 <?php
 /**
- * Tests for image-loading-optimization module load.php.
+ * Tests for image-loading-optimization module hooks.php.
  *
  * @package performance-lab
  * @group   image-loading-optimization
  */
 
-use PerformanceLab\Tests\TestCase\ImagesTestCase;
-
-class Image_Loading_Optimization_Load_Tests extends ImagesTestCase {
+class ILO_Hooks_Tests extends WP_UnitTestCase {
 
 	/**
 	 * Make sure the hook is added.
-	 *
-	 * @test
 	 */
-	public function it_is_hooking_output_buffering_at_template_include() {
+	public function test_hooking_output_buffering_at_template_include() {
 		$this->assertEquals( PHP_INT_MAX, has_filter( 'template_include', 'ilo_buffer_output' ) );
 	}
 
 	/**
 	 * Make output is buffered and that it is also filtered.
 	 *
-	 * @test
 	 * @covers ::ilo_buffer_output
 	 */
-	public function it_buffers_and_filters_output() {
+	public function test_buffering_and_filtering_output() {
 		$original = 'Hello World!';
 		$expected = '¡Hola Mundo!';
 
diff --git a/tests/modules/images/image-loading-optimization/optimization-tests.php b/tests/modules/images/image-loading-optimization/optimization-tests.php
new file mode 100644
index 000000000..e40f8dd85
--- /dev/null
+++ b/tests/modules/images/image-loading-optimization/optimization-tests.php
@@ -0,0 +1,531 @@
+<?php
+/**
+ * Tests for image-loading-optimization module optimization.php.
+ *
+ * @package performance-lab
+ * @group   image-loading-optimization
+ */
+
+class ILO_Optimization_Tests extends WP_UnitTestCase {
+
+	public function tear_down() {
+		parent::tear_down();
+		unset( $_SERVER['REQUEST_URI'] );
+	}
+
+	/**
+	 * Test ilo_maybe_add_template_output_buffer_filter().
+	 *
+	 * @covers ::ilo_maybe_add_template_output_buffer_filter
+	 */
+	public function test_ilo_maybe_add_template_output_buffer_filter() {
+		$this->assertFalse( has_filter( 'ilo_template_output_buffer', 'ilo_optimize_template_output_buffer' ) );
+
+		add_filter( 'ilo_can_optimize_response', '__return_false', 1 );
+		ilo_maybe_add_template_output_buffer_filter();
+		$this->assertFalse( ilo_can_optimize_response() );
+		$this->assertFalse( has_filter( 'ilo_template_output_buffer', 'ilo_optimize_template_output_buffer' ) );
+
+		add_filter( 'ilo_can_optimize_response', '__return_true', 2 );
+		$this->go_to( home_url( '/' ) );
+		$this->assertTrue( ilo_can_optimize_response() );
+		ilo_maybe_add_template_output_buffer_filter();
+		$this->assertSame( 10, has_filter( 'ilo_template_output_buffer', 'ilo_optimize_template_output_buffer' ) );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public function data_provider_test_ilo_can_optimize_response(): array {
+		return array(
+			'homepage'           => array(
+				'set_up'   => function () {
+					$this->go_to( home_url( '/' ) );
+				},
+				'expected' => true,
+			),
+			'homepage_filtered'  => array(
+				'set_up'   => function () {
+					$this->go_to( home_url( '/' ) );
+					add_filter( 'ilo_can_optimize_response', '__return_false' );
+				},
+				'expected' => false,
+			),
+			'search'             => array(
+				'set_up'   => function () {
+					self::factory()->post->create( array( 'post_title' => 'Hello' ) );
+					$this->go_to( home_url( '?s=Hello' ) );
+				},
+				'expected' => false,
+			),
+			'customizer_preview' => array(
+				'set_up'   => function () {
+					$this->go_to( home_url( '/' ) );
+					global $wp_customize;
+					/** @noinspection PhpIncludeInspection */
+					require_once ABSPATH . 'wp-includes/class-wp-customize-manager.php';
+					$wp_customize = new WP_Customize_Manager();
+					$wp_customize->start_previewing_theme();
+				},
+				'expected' => false,
+			),
+			'post_request'       => array(
+				'set_up'   => function () {
+					$this->go_to( home_url( '/' ) );
+					$_SERVER['REQUEST_METHOD'] = 'POST';
+				},
+				'expected' => false,
+			),
+		);
+	}
+
+	/**
+	 * Test ilo_can_optimize_response().
+	 *
+	 * @covers ::ilo_can_optimize_response
+	 *
+	 * @dataProvider data_provider_test_ilo_can_optimize_response
+	 */
+	public function test_ilo_can_optimize_response( Closure $set_up, bool $expected ) {
+		$set_up();
+		$this->assertSame( $expected, ilo_can_optimize_response() );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_provider_test_ilo_construct_preload_links(): array {
+		return array(
+			'no-lcp-image'                              => array(
+				'lcp_elements_by_minimum_viewport_widths' => array(
+					0 => false,
+				),
+				'expected'                                => '',
+			),
+			'one-non-responsive-lcp-image'              => array(
+				'lcp_elements_by_minimum_viewport_widths' => array(
+					0 => array(
+						'attributes' => array(
+							'src' => 'https://example.com/image.jpg',
+						),
+					),
+				),
+				'expected'                                => '
+					<link data-ilo-added-tag rel="preload" fetchpriority="high" as="image" href="https://example.com/image.jpg">
+				',
+			),
+			'one-responsive-lcp-image'                  => array(
+				'lcp_elements_by_minimum_viewport_widths' => array(
+					0 => array(
+						'attributes' => array(
+							'src'         => 'elva-fairy-800w.jpg',
+							'srcset'      => 'elva-fairy-480w.jpg 480w, elva-fairy-800w.jpg 800w',
+							'sizes'       => '(max-width: 600px) 480px, 800px',
+							'crossorigin' => 'anonymous',
+						),
+					),
+				),
+				'expected'                                => '
+					<link data-ilo-added-tag rel="preload" fetchpriority="high" as="image" imagesrcset="elva-fairy-480w.jpg 480w, elva-fairy-800w.jpg 800w" imagesizes="(max-width: 600px) 480px, 800px" crossorigin="anonymous">
+				',
+			),
+			'two-breakpoint-responsive-lcp-images'      => array(
+				'lcp_elements_by_minimum_viewport_widths' => array(
+					0   => array(
+						'attributes' => array(
+							'src'         => 'elva-fairy-800w.jpg',
+							'srcset'      => 'elva-fairy-480w.jpg 480w, elva-fairy-800w.jpg 800w',
+							'sizes'       => '(max-width: 600px) 480px, 800px',
+							'crossorigin' => 'anonymous',
+						),
+					),
+					601 => array(
+						'attributes' => array(
+							'src'         => 'alt-elva-fairy-800w.jpg',
+							'srcset'      => 'alt-elva-fairy-480w.jpg 480w, alt-elva-fairy-800w.jpg 800w',
+							'sizes'       => '(max-width: 600px) 480px, 800px',
+							'crossorigin' => 'anonymous',
+						),
+					),
+				),
+				'expected'                                => '
+					<link data-ilo-added-tag rel="preload" fetchpriority="high" as="image" imagesrcset="elva-fairy-480w.jpg 480w, elva-fairy-800w.jpg 800w" imagesizes="(max-width: 600px) 480px, 800px" crossorigin="anonymous" media="( min-width: 0px ) and ( max-width: 600px )">
+					<link data-ilo-added-tag rel="preload" fetchpriority="high" as="image" imagesrcset="alt-elva-fairy-480w.jpg 480w, alt-elva-fairy-800w.jpg 800w" imagesizes="(max-width: 600px) 480px, 800px" crossorigin="anonymous" media="( min-width: 601px )">
+				',
+			),
+			'two-non-consecutive-responsive-lcp-images' => array(
+				'lcp_elements_by_minimum_viewport_widths' => array(
+					0   => array(
+						'attributes' => array(
+							'src'         => 'elva-fairy-800w.jpg',
+							'srcset'      => 'elva-fairy-480w.jpg 480w, elva-fairy-800w.jpg 800w',
+							'sizes'       => '(max-width: 600px) 480px, 800px',
+							'crossorigin' => 'anonymous',
+						),
+					),
+					481 => false,
+					601 => array(
+						'attributes' => array(
+							'src'         => 'alt-elva-fairy-800w.jpg',
+							'srcset'      => 'alt-elva-fairy-480w.jpg 480w, alt-elva-fairy-800w.jpg 800w',
+							'sizes'       => '(max-width: 600px) 480px, 800px',
+							'crossorigin' => 'anonymous',
+						),
+					),
+				),
+				'expected'                                => '
+					<link data-ilo-added-tag rel="preload" fetchpriority="high" as="image" imagesrcset="elva-fairy-480w.jpg 480w, elva-fairy-800w.jpg 800w" imagesizes="(max-width: 600px) 480px, 800px" crossorigin="anonymous" media="( min-width: 0px ) and ( max-width: 480px )">
+					<link data-ilo-added-tag rel="preload" fetchpriority="high" as="image" imagesrcset="alt-elva-fairy-480w.jpg 480w, alt-elva-fairy-800w.jpg 800w" imagesizes="(max-width: 600px) 480px, 800px" crossorigin="anonymous" media="( min-width: 601px )">
+				',
+			),
+		);
+	}
+
+	/**
+	 * Test ilo_construct_preload_links().
+	 *
+	 * @covers ::ilo_construct_preload_links
+	 *
+	 * @dataProvider data_provider_test_ilo_construct_preload_links
+	 */
+	public function test_ilo_construct_preload_links( array $lcp_elements_by_minimum_viewport_widths, string $expected ) {
+		$this->assertSame(
+			$this->normalize_whitespace( $expected ),
+			$this->normalize_whitespace( ilo_construct_preload_links( $lcp_elements_by_minimum_viewport_widths ) )
+		);
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_provider_test_ilo_optimize_template_output_buffer(): array {
+		return array(
+			'no-url-metrics'                              => array(
+				'set_up'   => static function () {},
+				'buffer'   => '
+					<html lang="en">
+						<head>
+							<meta charset="utf-8">
+							<title>...</title>
+						</head>
+						<body>
+							<img src="https://example.com/foo.jpg" alt="Foo" width="1200" height="800" loading="lazy">
+						</body>
+					</html>
+				',
+				'expected' => '
+					<html lang="en">
+						<head>
+							<meta charset="utf-8">
+							<title>...</title>
+							<script type="module">/* import detect ... */</script>
+						</head>
+						<body>
+							<img data-ilo-xpath="/*[0][self::HTML]/*[1][self::BODY]/*[0][self::IMG]" src="https://example.com/foo.jpg" alt="Foo" width="1200" height="800" loading="lazy">
+						</body>
+					</html>
+				',
+			),
+
+			'common-lcp-image-with-fully-populated-sample-data' => array(
+				'set_up'   => function () {
+					$slug = ilo_get_url_metrics_slug( ilo_get_normalized_query_vars() );
+					$sample_size = ilo_get_url_metrics_breakpoint_sample_size();
+					foreach ( array_merge( ilo_get_breakpoint_max_widths(), array( 1000 ) ) as $viewport_width ) {
+						for ( $i = 0; $i < $sample_size; $i++ ) {
+							ilo_store_url_metric(
+								home_url( '/' ),
+								$slug,
+								$this->get_validated_url_metric(
+									$viewport_width,
+									array(
+										array(
+											'xpath' => '/*[0][self::HTML]/*[1][self::BODY]/*[0][self::IMG]',
+											'isLCP' => true,
+										),
+										array(
+											'xpath' => '/*[0][self::HTML]/*[1][self::BODY]/*[1][self::IMG]',
+											'isLCP' => false,
+										),
+									)
+								)
+							);
+						}
+					}
+				},
+				'buffer'   => '
+					<html lang="en">
+						<head>
+							<meta charset="utf-8">
+							<title>...</title>
+						</head>
+						<body>
+							<img src="https://example.com/foo.jpg" alt="Foo" width="1200" height="800" loading="lazy">
+							<img src="https://example.com/bar.jpg" alt="Bar" width="10" height="10" loading="lazy" fetchpriority="high">
+						</body>
+					</html>
+				',
+				'expected' => '
+					<html lang="en">
+						<head>
+							<meta charset="utf-8">
+							<title>...</title>
+							<link as="image" data-ilo-added-tag="" fetchpriority="high" href="https://example.com/foo.jpg" rel="preload"/>
+						</head>
+						<body>
+							<img src="https://example.com/foo.jpg" alt="Foo" width="1200" height="800" fetchpriority="high" data-ilo-added-fetchpriority data-ilo-removed-loading="lazy">
+							<img src="https://example.com/bar.jpg" alt="Bar" width="10" height="10" loading="lazy" data-ilo-removed-fetchpriority="high">
+						</body>
+					</html>
+				',
+			),
+
+			'fetch-priority-high-already-on-common-lcp-image-with-fully-populated-sample-data' => array(
+				'set_up'   => function () {
+					$slug = ilo_get_url_metrics_slug( ilo_get_normalized_query_vars() );
+					$sample_size = ilo_get_url_metrics_breakpoint_sample_size();
+					foreach ( array_merge( ilo_get_breakpoint_max_widths(), array( 1000 ) ) as $viewport_width ) {
+						for ( $i = 0; $i < $sample_size; $i++ ) {
+							ilo_store_url_metric(
+								home_url( '/' ),
+								$slug,
+								$this->get_validated_url_metric(
+									$viewport_width,
+									array(
+										array(
+											'isLCP' => true,
+											'xpath' => '/*[0][self::HTML]/*[1][self::BODY]/*[0][self::IMG]',
+										),
+									)
+								)
+							);
+						}
+					}
+				},
+				'buffer'   => '
+					<html lang="en">
+						<head>
+							<meta charset="utf-8">
+							<title>...</title>
+						</head>
+						<body>
+							<img src="https://example.com/foo.jpg" alt="Foo" width="1200" height="800" fetchpriority="high">
+						</body>
+					</html>
+				',
+				'expected' => '
+					<html lang="en">
+						<head>
+							<meta charset="utf-8">
+							<title>...</title>
+							<link as="image" data-ilo-added-tag="" fetchpriority="high" href="https://example.com/foo.jpg" rel="preload"/>
+						</head>
+						<body>
+							<img src="https://example.com/foo.jpg" alt="Foo" width="1200" height="800" fetchpriority="high" data-ilo-fetchpriority-already-added>
+						</body>
+					</html>
+				',
+			),
+
+			'url-metric-only-captured-for-one-breakpoint' => array(
+				'set_up'   => function () {
+					ilo_store_url_metric(
+						home_url( '/' ),
+						ilo_get_url_metrics_slug( ilo_get_normalized_query_vars() ),
+						$this->get_validated_url_metric(
+							400,
+							array(
+								array(
+									'isLCP' => true,
+									'xpath' => '/*[0][self::HTML]/*[1][self::BODY]/*[0][self::IMG]',
+								),
+							)
+						)
+					);
+				},
+				'buffer'   => '
+					<html lang="en">
+						<head>
+							<meta charset="utf-8">
+							<title>...</title>
+						</head>
+						<body>
+							<img src="https://example.com/foo.jpg" alt="Foo" width="1200" height="800">
+						</body>
+					</html>
+				',
+				'expected' => '
+					<html lang="en">
+						<head>
+							<meta charset="utf-8">
+							<title>...</title>
+							<link as="image" data-ilo-added-tag="" fetchpriority="high" href="https://example.com/foo.jpg" rel="preload"/>
+							<script type="module">/* import detect ... */</script>
+						</head>
+						<body>
+							<img data-ilo-xpath="/*[0][self::HTML]/*[1][self::BODY]/*[0][self::IMG]" src="https://example.com/foo.jpg" alt="Foo" width="1200" height="800">
+						</body>
+					</html>
+				',
+			),
+
+			'different-lcp-elements-for-different-breakpoints' => array(
+				'set_up'   => function () {
+					ilo_store_url_metric(
+						home_url( '/' ),
+						ilo_get_url_metrics_slug( ilo_get_normalized_query_vars() ),
+						$this->get_validated_url_metric(
+							400,
+							array(
+								array(
+									'isLCP' => true,
+									'xpath' => '/*[0][self::HTML]/*[1][self::BODY]/*[0][self::IMG]',
+								),
+								array(
+									'isLCP' => false,
+									'xpath' => '/*[0][self::HTML]/*[1][self::BODY]/*[1][self::IMG]',
+								),
+							)
+						)
+					);
+					ilo_store_url_metric(
+						home_url( '/' ),
+						ilo_get_url_metrics_slug( ilo_get_normalized_query_vars() ),
+						$this->get_validated_url_metric(
+							800,
+							array(
+								array(
+									'isLCP' => false,
+									'xpath' => '/*[0][self::HTML]/*[1][self::BODY]/*[0][self::IMG]',
+								),
+								array(
+									'isLCP' => true,
+									'xpath' => '/*[0][self::HTML]/*[1][self::BODY]/*[1][self::IMG]',
+								),
+							)
+						)
+					);
+				},
+				'buffer'   => '
+					<html lang="en">
+						<head>
+							<meta charset="utf-8">
+							<title>...</title>
+						</head>
+						<body>
+							<img src="https://example.com/mobile-logo.png" alt="Mobile Logo" width="600" height="600">
+							<img src="https://example.com/desktop-logo.png" alt="Desktop Logo" width="600" height="600">
+						</body>
+					</html>
+				',
+				'expected' => '
+					<html lang="en">
+						<head>
+							<meta charset="utf-8">
+							<title>...</title>
+							<link as="image" data-ilo-added-tag="" fetchpriority="high" href="https://example.com/mobile-logo.png" media="( min-width: 0px ) and ( max-width: 782px )" rel="preload">
+							<link as="image" data-ilo-added-tag="" fetchpriority="high" href="https://example.com/desktop-logo.png" media="( min-width: 783px )" rel="preload">
+							<script type="module">/* import detect ... */</script>
+						</head>
+						<body>
+							<img alt="Mobile Logo" data-ilo-xpath="/*[0][self::HTML]/*[1][self::BODY]/*[0][self::IMG]" height="600" src="https://example.com/mobile-logo.png" width="600"/>
+							<img alt="Desktop Logo" data-ilo-xpath="/*[0][self::HTML]/*[1][self::BODY]/*[1][self::IMG]" height="600" src="https://example.com/desktop-logo.png" width="600"/>
+						</body>
+					</html>
+				',
+			),
+
+		);
+	}
+
+	/**
+	 * Test ilo_optimize_template_output_buffer().
+	 *
+	 * @covers ::ilo_optimize_template_output_buffer
+	 *
+	 * @dataProvider data_provider_test_ilo_optimize_template_output_buffer
+	 */
+	public function test_ilo_optimize_template_output_buffer( Closure $set_up, string $buffer, string $expected ) {
+		$set_up();
+		$this->assertEquals(
+			$this->parse_html_document( $expected ),
+			$this->parse_html_document( ilo_optimize_template_output_buffer( $buffer ) )
+		);
+	}
+
+	/**
+	 * Normalizes whitespace.
+	 *
+	 * @param string $str String to normalize.
+	 * @return string Normalized string.
+	 */
+	private function normalize_whitespace( string $str ): string {
+		return preg_replace( '/\s+/', ' ', trim( $str ) );
+	}
+
+	/**
+	 * Gets a validated URL metric.
+	 *
+	 * @param int $viewport_width Viewport width for the URL metric.
+	 * @return array URL metric.
+	 */
+	private function get_validated_url_metric( int $viewport_width, array $elements = array() ): array {
+		return array(
+			'viewport' => array(
+				'width'  => $viewport_width,
+				'height' => 800,
+			),
+			'elements' => array_map(
+				static function ( array $element ): array {
+					return array_merge(
+						array(
+							'isLCPCandidate'    => true,
+							'intersectionRatio' => 1,
+						),
+						$element
+					);
+				},
+				$elements
+			),
+		);
+	}
+
+	/**
+	 * Parse an HTML markup fragment and normalize for comparison.
+	 *
+	 * @param string $markup Markup.
+	 * @return DOMDocument Document containing the normalized markup fragment.
+	 */
+	protected function parse_html_document( string $markup ): DOMDocument {
+		$dom = new DOMDocument();
+		$dom->loadHTML( trim( $markup ) );
+
+		// Remove all whitespace nodes.
+		$xpath = new DOMXPath( $dom );
+		foreach ( $xpath->query( '//text()' ) as $node ) {
+			/** @var DOMText $node */
+			if ( preg_match( '/^\s+$/', $node->nodeValue ) ) {
+				$node->nodeValue = '';
+			}
+		}
+
+		// Insert a newline before each node to make the diff easier to read.
+		foreach ( $xpath->query( '/html//*' ) as $node ) {
+			/** @var DOMElement $node */
+			$node->parentNode->insertBefore( $dom->createTextNode( "\n" ), $node );
+		}
+
+		// Normalize contents of module script output by ilo_get_detection_script().
+		foreach ( $xpath->query( '//script[ contains( text(), "import detect" ) ]' ) as $script ) {
+			$script->textContent = '/* import detect ... */';
+		}
+
+		return $dom;
+	}
+}
diff --git a/tests/modules/images/image-loading-optimization/storage/data-tests.php b/tests/modules/images/image-loading-optimization/storage/data-tests.php
new file mode 100644
index 000000000..d14d47740
--- /dev/null
+++ b/tests/modules/images/image-loading-optimization/storage/data-tests.php
@@ -0,0 +1,563 @@
+<?php
+/**
+ * Tests for image-loading-optimization module storage/data.php.
+ *
+ * @package performance-lab
+ * @group   image-loading-optimization
+ */
+
+class ILO_Storage_Data_Tests extends WP_UnitTestCase {
+
+	public function tear_down() {
+		unset(
+			$GLOBALS['wp_customize'],
+			$_SERVER['REQUEST_URI']
+		);
+		parent::tear_down();
+	}
+
+	/**
+	 * Test ilo_get_url_metric_freshness_ttl().
+	 *
+	 * @covers ::ilo_get_url_metric_freshness_ttl
+	 */
+	public function test_ilo_get_url_metric_freshness_ttl() {
+		$this->assertSame( DAY_IN_SECONDS, ilo_get_url_metric_freshness_ttl() );
+
+		add_filter(
+			'ilo_url_metric_freshness_ttl',
+			static function (): int {
+				return HOUR_IN_SECONDS;
+			}
+		);
+
+		$this->assertSame( HOUR_IN_SECONDS, ilo_get_url_metric_freshness_ttl() );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public function data_provider_test_ilo_get_normalized_query_vars(): array {
+		return array(
+			'homepage'     => array(
+				'set_up' => function (): array {
+					$this->go_to( home_url( '/' ) );
+					return array();
+				},
+			),
+			'post'         => array(
+				'set_up' => function (): array {
+					$post_id = self::factory()->post->create();
+					$this->go_to( get_permalink( $post_id ) );
+					return array( 'p' => (string) $post_id );
+				},
+			),
+			'date-archive' => array(
+				'set_up' => function (): array {
+					$post_id = self::factory()->post->create();
+					$date = get_post_datetime( $post_id );
+
+					$this->go_to(
+						add_query_arg(
+							array(
+								'day'      => $date->format( 'j' ),
+								'year'     => $date->format( 'Y' ),
+								'monthnum' => $date->format( 'm' ),
+								'bogus'    => 'ignore me',
+							),
+							home_url()
+						)
+					);
+					return array(
+						'year'     => $date->format( 'Y' ),
+						'monthnum' => $date->format( 'm' ),
+						'day'      => $date->format( 'j' ),
+					);
+				},
+			),
+			'404'          => array(
+				'set_up' => function () {
+					$this->go_to( home_url( '/?p=1000000' ) );
+					return array( 'error' => 404 );
+				},
+			),
+		);
+	}
+
+	/**
+	 * Test ilo_get_normalized_query_vars().
+	 *
+	 * @covers ::ilo_get_normalized_query_vars
+	 *
+	 * @dataProvider data_provider_test_ilo_get_normalized_query_vars
+	 */
+	public function test_ilo_get_normalized_query_vars( Closure $set_up ) {
+		$expected = $set_up();
+		$this->assertSame( $expected, ilo_get_normalized_query_vars() );
+	}
+
+	/**
+	 * Test ilo_get_url_metrics_slug().
+	 *
+	 * @covers ::ilo_get_url_metrics_slug
+	 */
+	public function test_ilo_get_url_metrics_slug() {
+		$first  = ilo_get_url_metrics_slug( array() );
+		$second = ilo_get_url_metrics_slug( array( 'p' => 1 ) );
+		$this->assertNotEquals( $second, $first );
+		foreach ( array( $first, $second ) as $slug ) {
+			$this->assertMatchesRegularExpression( '/^[0-9a-f]{32}$/', $slug );
+		}
+	}
+
+	/**
+	 * Test ilo_get_url_metrics_storage_nonce().
+	 *
+	 * @covers ::ilo_get_url_metrics_storage_nonce
+	 * @covers ::ilo_verify_url_metrics_storage_nonce
+	 */
+	public function test_ilo_get_url_metrics_storage_nonce_and_ilo_verify_url_metrics_storage_nonce() {
+		$user_id = self::factory()->user->create();
+
+		$nonce_life_actions = array();
+		add_filter(
+			'nonce_life',
+			static function ( int $life, string $action ) use ( &$nonce_life_actions ): int {
+				$nonce_life_actions[] = $action;
+				return $life;
+			},
+			10,
+			2
+		);
+
+		// Create first nonce for unauthenticated user.
+		$slug   = ilo_get_url_metrics_slug( array() );
+		$nonce1 = ilo_get_url_metrics_storage_nonce( $slug );
+		$this->assertMatchesRegularExpression( '/^[0-9a-f]{10}$/', $nonce1 );
+		$this->assertTrue( ilo_verify_url_metrics_storage_nonce( $nonce1, $slug ) );
+		$this->assertCount( 2, $nonce_life_actions );
+
+		// Create second nonce for unauthenticated user.
+		$nonce2 = ilo_get_url_metrics_storage_nonce( $slug );
+		$this->assertSame( $nonce1, $nonce2 );
+		$this->assertCount( 3, $nonce_life_actions );
+
+		// Create third nonce, this time for authenticated user.
+		wp_set_current_user( $user_id );
+		$nonce3 = ilo_get_url_metrics_storage_nonce( $slug );
+		$this->assertNotEquals( $nonce3, $nonce2 );
+		$this->assertFalse( ilo_verify_url_metrics_storage_nonce( $nonce1, $slug ) );
+		$this->assertTrue( ilo_verify_url_metrics_storage_nonce( $nonce3, $slug ) );
+		$this->assertCount( 6, $nonce_life_actions );
+
+		foreach ( $nonce_life_actions as $nonce_life_action ) {
+			$this->assertSame( "store_url_metrics:{$slug}", $nonce_life_action );
+		}
+	}
+
+	public function data_provider_sample_size_and_breakpoints(): array {
+		return array(
+			'3 sample size and 2 breakpoints' => array(
+				'sample_size'     => 3,
+				'breakpoints'     => array( 480, 782 ),
+				'viewport_widths' => array( 400, 600, 800 ),
+			),
+			'1 sample size and 1 breakpoint'  => array(
+				'sample_size'     => 1,
+				'breakpoints'     => array( 480 ),
+				'viewport_widths' => array( 400, 800 ),
+			),
+		);
+	}
+
+	/**
+	 * Test ilo_unshift_url_metrics().
+	 *
+	 * @covers ::ilo_unshift_url_metrics
+	 *
+	 * @dataProvider data_provider_sample_size_and_breakpoints
+	 */
+	public function test_ilo_unshift_url_metrics( int $sample_size, array $breakpoints, array $viewport_widths ) {
+		$old_timestamp = 1701978742;
+
+		// Fully populate the sample size for the breakpoints.
+		$all_url_metrics = array();
+		foreach ( $viewport_widths as $viewport_width ) {
+			for ( $i = 0; $i < $sample_size; $i++ ) {
+				$all_url_metrics = ilo_unshift_url_metrics(
+					$all_url_metrics,
+					$this->get_validated_url_metric( $viewport_width ),
+					$breakpoints,
+					$sample_size
+				);
+			}
+		}
+		$max_possible_url_metrics_count = $sample_size * ( count( $breakpoints ) + 1 );
+		$this->assertCount(
+			$max_possible_url_metrics_count,
+			$all_url_metrics,
+			sprintf( 'Expected there to be exactly sample size (%d) times the number of breakpoint groups (which is %d + 1)', $sample_size, count( $breakpoints ) )
+		);
+
+		// Make sure that ilo_unshift_url_metrics() added a timestamp and then force them to all be old.
+		$all_url_metrics = array_map(
+			function ( $url_metric ) use ( $old_timestamp ) {
+				$this->assertArrayHasKey( 'timestamp', $url_metric, 'Expected a timestamp to have been added to a URL metric.' );
+				$url_metric['timestamp'] = $old_timestamp;
+				return $url_metric;
+			},
+			$all_url_metrics
+		);
+
+		// Try adding one URL metric for each breakpoint group.
+		foreach ( $viewport_widths as $viewport_width ) {
+			$all_url_metrics = ilo_unshift_url_metrics(
+				$all_url_metrics,
+				$this->get_validated_url_metric( $viewport_width ),
+				$breakpoints,
+				$sample_size
+			);
+		}
+		$this->assertCount(
+			$max_possible_url_metrics_count,
+			$all_url_metrics,
+			'Expected the total count of URL metrics to not exceed the multiple of the sample size.'
+		);
+		$new_count = 0;
+		foreach ( $all_url_metrics as $url_metric ) {
+			if ( $url_metric['timestamp'] > $old_timestamp ) {
+				++$new_count;
+			}
+		}
+		$this->assertGreaterThan( 0, $new_count, 'Expected there to be at least one new URL metric.' );
+		$this->assertSame( count( $viewport_widths ), $new_count, 'Expected the new URL metrics to all have been added.' );
+	}
+
+	/**
+	 * Test ilo_get_breakpoint_max_widths().
+	 *
+	 * @covers ::ilo_get_breakpoint_max_widths
+	 */
+	public function test_ilo_get_breakpoint_max_widths() {
+		$this->assertSame(
+			array( 480, 600, 782 ),
+			ilo_get_breakpoint_max_widths()
+		);
+
+		$filtered_breakpoints = array( 2000, 500, '1000', 3000 );
+
+		add_filter(
+			'ilo_breakpoint_max_widths',
+			static function () use ( $filtered_breakpoints ) {
+				return $filtered_breakpoints;
+			}
+		);
+
+		$filtered_breakpoints = array_map( 'intval', $filtered_breakpoints );
+		sort( $filtered_breakpoints );
+		$this->assertSame( $filtered_breakpoints, ilo_get_breakpoint_max_widths() );
+	}
+
+	/**
+	 * Test ilo_get_url_metrics_breakpoint_sample_size().
+	 *
+	 * @covers ::ilo_get_url_metrics_breakpoint_sample_size
+	 */
+	public function test_ilo_get_url_metrics_breakpoint_sample_size() {
+		$this->assertSame( 3, ilo_get_url_metrics_breakpoint_sample_size() );
+
+		add_filter(
+			'ilo_url_metrics_breakpoint_sample_size',
+			static function () {
+				return '1';
+			}
+		);
+
+		$this->assertSame( 1, ilo_get_url_metrics_breakpoint_sample_size() );
+	}
+
+	public function data_provider_test_ilo_group_url_metrics_by_breakpoint(): array {
+		return array(
+			'2-breakpoints-and-3-viewport-widths' => array(
+				'breakpoints'     => array( 480, 640 ),
+				'viewport_widths' => array( 400, 480, 800 ),
+			),
+			'1-breakpoint-and-4-viewport-widths'  => array(
+				'breakpoints'     => array( 480 ),
+				'viewport_widths' => array( 400, 600, 800, 1000 ),
+			),
+		);
+	}
+
+	/**
+	 * Test ilo_group_url_metrics_by_breakpoint().
+	 *
+	 * @covers ::ilo_group_url_metrics_by_breakpoint
+	 *
+	 * @dataProvider data_provider_test_ilo_group_url_metrics_by_breakpoint
+	 */
+	public function test_ilo_group_url_metrics_by_breakpoint( array $breakpoints, array $viewport_widths ) {
+		$url_metrics = array_map(
+			function ( $viewport_width ) {
+				return $this->get_validated_url_metric( $viewport_width );
+			},
+			$viewport_widths
+		);
+
+		$grouped_url_metrics = ilo_group_url_metrics_by_breakpoint( $url_metrics, $breakpoints );
+		$this->assertCount( count( $breakpoints ) + 1, $grouped_url_metrics, 'Expected number of breakpoint groups to always be one greater than the number of breakpoints.' );
+		$minimum_viewport_widths = array_keys( $grouped_url_metrics );
+		$this->assertSame( 0, array_shift( $minimum_viewport_widths ), 'Expected the first minimum viewport width to always be zero.' );
+		foreach ( $breakpoints as $breakpoint ) {
+			$this->assertSame( $breakpoint + 1, array_shift( $minimum_viewport_widths ) );
+		}
+
+		$minimum_viewport_widths = array_keys( $grouped_url_metrics );
+		for ( $i = 0, $len = count( $minimum_viewport_widths ); $i < $len; $i++ ) {
+			$minimum_viewport_width = $minimum_viewport_widths[ $i ];
+			$maximum_viewport_width = $minimum_viewport_widths[ $i + 1 ] ?? null;
+			if ( 0 === $i ) {
+				$this->assertSame( 0, $minimum_viewport_width );
+			} else {
+				$this->assertGreaterThan( 0, $minimum_viewport_width );
+			}
+			if ( isset( $maximum_viewport_width ) ) {
+				$this->assertLessThan( $maximum_viewport_width, $minimum_viewport_width );
+			}
+
+			$this->assertIsArray( $grouped_url_metrics[ $minimum_viewport_width ] );
+			foreach ( $grouped_url_metrics[ $minimum_viewport_width ] as $url_metric ) {
+				$this->assertGreaterThanOrEqual( $minimum_viewport_width, $url_metric['viewport']['width'] );
+				if ( isset( $maximum_viewport_width ) ) {
+					$this->assertLessThanOrEqual( $maximum_viewport_width, $url_metric['viewport']['width'] );
+				}
+			}
+		}
+	}
+
+	public function data_provider_test_ilo_get_lcp_elements_by_minimum_viewport_widths(): array {
+		return array(
+			'common_lcp_element_across_breakpoints'    => array(
+				'grouped_url_metrics'         => array(
+					0   => array(
+						$this->get_validated_url_metric( 400, array( 'HTML', 'BODY', 'FIGURE', 'IMG' ) ),
+						$this->get_validated_url_metric( 500, array( 'HTML', 'BODY', 'DIV', 'IMG' ) ), // Ignored since less common than the other two.
+						$this->get_validated_url_metric( 599, array( 'HTML', 'BODY', 'FIGURE', 'IMG' ) ),
+					),
+					600 => array(
+						$this->get_validated_url_metric( 600, array( 'HTML', 'BODY', 'FIGURE', 'IMG' ) ),
+						$this->get_validated_url_metric( 700, array( 'HTML', 'BODY', 'FIGURE', 'IMG' ) ),
+					),
+					800 => array(
+						$this->get_validated_url_metric( 900, array( 'HTML', 'BODY', 'FIGURE', 'IMG' ) ),
+					),
+				),
+				'expected_lcp_element_xpaths' => array(
+					0 => $this->get_xpath( 'HTML', 'BODY', 'FIGURE', 'IMG' ),
+				),
+			),
+			'different_lcp_elements_across_breakpoint' => array(
+				'grouped_url_metrics'         => array(
+					0   => array(
+						$this->get_validated_url_metric( 400, array( 'HTML', 'BODY', 'FIGURE', 'IMG' ) ),
+						$this->get_validated_url_metric( 500, array( 'HTML', 'BODY', 'DIV', 'IMG' ) ), // Ignored since less common than the other two.
+						$this->get_validated_url_metric( 599, array( 'HTML', 'BODY', 'FIGURE', 'IMG' ) ),
+					),
+					600 => array(
+						$this->get_validated_url_metric( 800, array( 'HTML', 'BODY', 'MAIN', 'IMG' ) ),
+						$this->get_validated_url_metric( 900, array( 'HTML', 'BODY', 'MAIN', 'IMG' ) ),
+					),
+				),
+				'expected_lcp_element_xpaths' => array(
+					0   => $this->get_xpath( 'HTML', 'BODY', 'FIGURE', 'IMG' ),
+					600 => $this->get_xpath( 'HTML', 'BODY', 'MAIN', 'IMG' ),
+				),
+			),
+			'same_lcp_element_across_non_consecutive_breakpoints' => array(
+				'grouped_url_metrics'         => array(
+					0   => array(
+						$this->get_validated_url_metric( 300, array( 'HTML', 'BODY', 'MAIN', 'IMG' ) ),
+					),
+					400 => array(
+						$this->get_validated_url_metric( 500, array( 'HTML', 'BODY', 'HEADER', 'IMG' ), false ),
+					),
+					600 => array(
+						$this->get_validated_url_metric( 800, array( 'HTML', 'BODY', 'MAIN', 'IMG' ) ),
+						$this->get_validated_url_metric( 900, array( 'HTML', 'BODY', 'MAIN', 'IMG' ) ),
+					),
+				),
+				'expected_lcp_element_xpaths' => array(
+					0   => $this->get_xpath( 'HTML', 'BODY', 'MAIN', 'IMG' ),
+					400 => false, // The (image) element is either not visible at this breakpoint or it is not LCP element.
+					600 => $this->get_xpath( 'HTML', 'BODY', 'MAIN', 'IMG' ),
+				),
+			),
+			'no_lcp_image_elements'                    => array(
+				'grouped_url_metrics'         => array(
+					0   => array(
+						$this->get_validated_url_metric( 300, array( 'HTML', 'BODY', 'IMG' ), false ),
+					),
+					600 => array(
+						$this->get_validated_url_metric( 300, array( 'HTML', 'BODY', 'IMG' ), false ),
+					),
+				),
+				'expected_lcp_element_xpaths' => array(
+					0 => false,
+				),
+			),
+		);
+	}
+
+	/**
+	 * Test ilo_get_lcp_elements_by_minimum_viewport_widths().
+	 *
+	 * @covers ::ilo_get_lcp_elements_by_minimum_viewport_widths
+	 * @dataProvider data_provider_test_ilo_get_lcp_elements_by_minimum_viewport_widths
+	 */
+	public function test_ilo_get_lcp_elements_by_minimum_viewport_widths( array $grouped_url_metrics, array $expected_lcp_element_xpaths ) {
+		$lcp_elements_by_minimum_viewport_widths = ilo_get_lcp_elements_by_minimum_viewport_widths( $grouped_url_metrics );
+
+		$lcp_element_xpaths_by_minimum_viewport_widths = array();
+		foreach ( $lcp_elements_by_minimum_viewport_widths as $minimum_viewport_width => $lcp_element ) {
+			$this->assertTrue( is_array( $lcp_element ) || false === $lcp_element );
+			if ( is_array( $lcp_element ) ) {
+				$this->assertTrue( $lcp_element['isLCP'] );
+				$this->assertTrue( $lcp_element['isLCPCandidate'] );
+				$this->assertIsString( $lcp_element['xpath'] );
+				$this->assertIsNumeric( $lcp_element['intersectionRatio'] );
+				$lcp_element_xpaths_by_minimum_viewport_widths[ $minimum_viewport_width ] = $lcp_element['xpath'];
+			} else {
+				$lcp_element_xpaths_by_minimum_viewport_widths[ $minimum_viewport_width ] = false;
+			}
+		}
+
+		$this->assertSame( $expected_lcp_element_xpaths, $lcp_element_xpaths_by_minimum_viewport_widths );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_provider_test_ilo_get_needed_minimum_viewport_widths(): array {
+		$current_time = microtime( true );
+
+		$none_needed_data = array(
+			'url_metrics'           => ( function () use ( $current_time ): array {
+				return array_merge(
+					array_fill(
+						0,
+						3,
+						array_merge( $this->get_validated_url_metric( 400 ), array( 'timestamp' => $current_time ) )
+					),
+					array_fill(
+						0,
+						3,
+						array_merge( $this->get_validated_url_metric( 600 ), array( 'timestamp' => $current_time ) )
+					)
+				);
+			} )(),
+			'current_time'          => $current_time,
+			'breakpoint_max_widths' => array( 480 ),
+			'sample_size'           => 3,
+			'freshness_ttl'         => HOUR_IN_SECONDS,
+		);
+
+		return array(
+			'none-needed'            => array_merge(
+				$none_needed_data,
+				array(
+					'expected' => array(
+						array( 0, false ),
+						array( 481, false ),
+					),
+				)
+			),
+
+			'not-enough-url-metrics' => array_merge(
+				$none_needed_data,
+				array(
+					'sample_size' => $none_needed_data['sample_size'] + 1,
+				),
+				array(
+					'expected' => array(
+						array( 0, true ),
+						array( 481, true ),
+					),
+				)
+			),
+
+			'url-metric-too-old'     => array_merge(
+				( static function ( $data ): array {
+					$data['url_metrics'][0]['timestamp'] -= $data['freshness_ttl'] + 1;
+					return $data;
+				} )( $none_needed_data ),
+				array(
+					'expected' => array(
+						array( 0, true ),
+						array( 481, false ),
+					),
+				)
+			),
+		);
+	}
+
+	/**
+	 * Test ilo_get_needed_minimum_viewport_widths().
+	 *
+	 * @covers ::ilo_get_needed_minimum_viewport_widths
+	 *
+	 * @dataProvider data_provider_test_ilo_get_needed_minimum_viewport_widths
+	 */
+	public function test_ilo_get_needed_minimum_viewport_widths( array $url_metrics, float $current_time, array $breakpoint_max_widths, int $sample_size, int $freshness_ttl, array $expected ) {
+		$this->assertSame(
+			$expected,
+			ilo_get_needed_minimum_viewport_widths( $url_metrics, $current_time, $breakpoint_max_widths, $sample_size, $freshness_ttl )
+		);
+	}
+
+	/**
+	 * Gets a validated URL metric for testing.
+	 *
+	 * @param int      $viewport_width Viewport width.
+	 * @param string[] $breadcrumbs    Breadcrumb tags.
+	 * @param bool     $is_lcp         Whether LCP.
+	 * @return array Validated URL metric.
+	 */
+	private function get_validated_url_metric( int $viewport_width = 480, array $breadcrumbs = array( 'HTML', 'BODY', 'IMG' ), bool $is_lcp = true ): array {
+		return array(
+			'viewport' => array(
+				'width'  => $viewport_width,
+				'height' => 640,
+			),
+			'elements' => array(
+				array(
+					'isLCP'             => $is_lcp,
+					'isLCPCandidate'    => $is_lcp,
+					'xpath'             => $this->get_xpath( ...$breadcrumbs ),
+					'intersectionRatio' => 1,
+				),
+			),
+		);
+	}
+
+	/**
+	 * Gets sample XPath.
+	 *
+	 * @param string ...$breadcrumbs List of tags.
+	 * @return string XPath.
+	 */
+	private function get_xpath( ...$breadcrumbs ): string {
+		return implode(
+			'',
+			array_map(
+				static function ( $tag ) {
+					return sprintf( '/*[0][self::%s]', strtoupper( $tag ) );
+				},
+				$breadcrumbs
+			)
+		);
+	}
+}
diff --git a/tests/modules/images/image-loading-optimization/storage/lock-tests.php b/tests/modules/images/image-loading-optimization/storage/lock-tests.php
new file mode 100644
index 000000000..dfedb566d
--- /dev/null
+++ b/tests/modules/images/image-loading-optimization/storage/lock-tests.php
@@ -0,0 +1,129 @@
+<?php
+/**
+ * Tests for image-loading-optimization module storage/lock.php.
+ *
+ * @package performance-lab
+ * @group   image-loading-optimization
+ */
+
+class ILO_Storage_Lock_Tests extends WP_UnitTestCase {
+
+	/**
+	 * Tear down.
+	 */
+	public function tear_down() {
+		unset( $_SERVER['HTTP_X_FORWARDED_FOR'] );
+		parent::tear_down();
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array<string, array{set_up: Closure, expected: int}>
+	 */
+	public function data_provider_ilo_get_url_metric_storage_lock_ttl(): array {
+		return array(
+			'unfiltered'        => array(
+				'set_up'   => static function () {},
+				'expected' => MINUTE_IN_SECONDS,
+			),
+			'filtered_hour'     => array(
+				'set_up'   => static function () {
+					add_filter(
+						'ilo_url_metric_storage_lock_ttl',
+						static function (): int {
+							return HOUR_IN_SECONDS;
+						}
+					);
+				},
+				'expected' => HOUR_IN_SECONDS,
+			),
+			'filtered_negative' => array(
+				'set_up'   => static function () {
+					add_filter(
+						'ilo_url_metric_storage_lock_ttl',
+						static function (): int {
+							return -100;
+						}
+					);
+				},
+				'expected' => 0,
+			),
+		);
+	}
+
+	/**
+	 * Test ilo_get_url_metric_storage_lock_ttl().
+	 *
+	 * @covers ::ilo_get_url_metric_storage_lock_ttl
+	 *
+	 * @dataProvider data_provider_ilo_get_url_metric_storage_lock_ttl
+	 *
+	 * @param Closure $set_up   Set up.
+	 * @param int     $expected Expected value.
+	 */
+	public function test_ilo_get_url_metric_storage_lock_ttl( Closure $set_up, int $expected ) {
+		$set_up();
+		$this->assertSame( $expected, ilo_get_url_metric_storage_lock_ttl() );
+	}
+
+	/**
+	 * Test ilo_get_url_metric_storage_lock_transient_key().
+	 *
+	 * @covers ::ilo_get_url_metric_storage_lock_transient_key
+	 */
+	public function test_ilo_get_url_metric_storage_lock_transient_key() {
+		unset( $_SERVER['REMOTE_ADDR'], $_SERVER['HTTP_X_FORWARDED_FOR'] );
+
+		$_SERVER['REMOTE_ADDR'] = '127.0.0.1';
+		$first_key              = ilo_get_url_metric_storage_lock_transient_key();
+		$this->assertStringStartsWith( 'url_metrics_storage_lock_', $first_key );
+
+		$_SERVER['HTTP_X_FORWARDED_FOR'] = '127.0.0.2';
+		$second_key                      = ilo_get_url_metric_storage_lock_transient_key();
+		$this->assertStringStartsWith( 'url_metrics_storage_lock_', $second_key );
+
+		$this->assertNotEquals( $second_key, $first_key, 'Expected setting HTTP_X_FORWARDED_FOR header to take precedence over REMOTE_ADDR.' );
+	}
+
+	/**
+	 * Test ilo_set_url_metric_storage_lock() and ilo_is_url_metric_storage_locked().
+	 *
+	 * @covers ::ilo_set_url_metric_storage_lock
+	 * @covers ::ilo_is_url_metric_storage_locked
+	 */
+	public function test_ilo_set_url_metric_storage_lock_and_ilo_is_url_metric_storage_locked() {
+		$key = ilo_get_url_metric_storage_lock_transient_key();
+		$ttl = ilo_get_url_metric_storage_lock_ttl();
+
+		$transient_value      = null;
+		$transient_expiration = null;
+		add_action(
+			"set_transient_{$key}",
+			static function ( $filtered_value, $filtered_expiration ) use ( &$transient_value, &$transient_expiration ) {
+				$transient_value      = $filtered_value;
+				$transient_expiration = $filtered_expiration;
+				return $filtered_value;
+			},
+			10,
+			2
+		);
+
+		// Set the lock.
+		ilo_set_url_metric_storage_lock();
+		$this->assertSame( $ttl, $transient_expiration );
+		$this->assertLessThanOrEqual( microtime( true ), $transient_value );
+		$this->assertEquals( $transient_value, get_transient( $key ) );
+		$this->assertTrue( ilo_is_url_metric_storage_locked() );
+
+		// Simulate expired lock.
+		set_transient( $key, microtime( true ) - HOUR_IN_SECONDS );
+		$this->assertFalse( ilo_is_url_metric_storage_locked() );
+
+		// Clear the lock.
+		add_filter( 'ilo_url_metric_storage_lock_ttl', '__return_zero' );
+		ilo_set_url_metric_storage_lock();
+		$this->assertFalse( get_transient( $key ) );
+		$this->assertFalse( ilo_is_url_metric_storage_locked() );
+	}
+}
diff --git a/tests/modules/images/image-loading-optimization/storage/post-type-tests.php b/tests/modules/images/image-loading-optimization/storage/post-type-tests.php
new file mode 100644
index 000000000..15b298016
--- /dev/null
+++ b/tests/modules/images/image-loading-optimization/storage/post-type-tests.php
@@ -0,0 +1,159 @@
+<?php
+/**
+ * Tests for image-loading-optimization module storage/post-type.php.
+ *
+ * @package performance-lab
+ * @group   image-loading-optimization
+ */
+
+class ILO_Storage_Post_Type_Tests extends WP_UnitTestCase {
+
+	/**
+	 * Test ilo_register_url_metrics_post_type().
+	 *
+	 * @covers ::ilo_register_url_metrics_post_type
+	 */
+	public function test_ilo_register_url_metrics_post_type() {
+		$this->assertSame( 10, has_action( 'init', 'ilo_register_url_metrics_post_type' ) );
+		$post_type_object = get_post_type_object( ILO_URL_METRICS_POST_TYPE );
+		$this->assertInstanceOf( WP_Post_Type::class, $post_type_object );
+		$this->assertFalse( $post_type_object->public );
+	}
+
+	/**
+	 * Test ilo_get_url_metrics_post() when there is no post.
+	 *
+	 * @covers ::ilo_get_url_metrics_post
+	 */
+	public function test_ilo_get_url_metrics_post_when_absent() {
+		$slug = ilo_get_url_metrics_slug( array( 'p' => '1' ) );
+		$this->assertNull( ilo_get_url_metrics_post( $slug ) );
+	}
+
+	/**
+	 * Test ilo_get_url_metrics_post() when there is a post.
+	 *
+	 * @covers ::ilo_get_url_metrics_post
+	 */
+	public function test_ilo_get_url_metrics_post_when_present() {
+		$slug = ilo_get_url_metrics_slug( array( 'p' => '1' ) );
+
+		$post_id = self::factory()->post->create(
+			array(
+				'post_type' => ILO_URL_METRICS_POST_TYPE,
+				'post_name' => $slug,
+			)
+		);
+
+		$post = ilo_get_url_metrics_post( $slug );
+		$this->assertInstanceOf( WP_Post::class, $post );
+		$this->assertSame( $post_id, $post->ID );
+	}
+
+	/**
+	 * Data provider for test_ilo_parse_stored_url_metrics.
+	 *
+	 * @return array<string, array{post_content: string, expected_value: array}>
+	 */
+	public function data_provider_test_ilo_parse_stored_url_metrics(): array {
+		$valid_content = array(
+			array(
+				'viewport' => array(
+					'width'  => 640,
+					'height' => 480,
+				),
+				'elements' => array(),
+			),
+		);
+
+		return array(
+			'malformed_json' => array(
+				'post_content'   => '{"bad":',
+				'expected_value' => array(),
+			),
+			'not_array_json' => array(
+				'post_content'   => '{"cool":"beans"}',
+				'expected_value' => array(),
+			),
+			'missing_keys'   => array(
+				'post_content'   => '[{},{},{}]',
+				'expected_value' => array(),
+			),
+			'valid'          => array(
+				'post_content'   => wp_json_encode( $valid_content ),
+				'expected_value' => $valid_content,
+			),
+		);
+	}
+
+	/**
+	 * Test ilo_parse_stored_url_metrics().
+	 *
+	 * @covers ::ilo_parse_stored_url_metrics
+	 *
+	 * @dataProvider data_provider_test_ilo_parse_stored_url_metrics
+	 */
+	public function test_ilo_parse_stored_url_metrics( string $post_content, array $expected_value ) {
+		$post = self::factory()->post->create_and_get(
+			array(
+				'post_type'    => ILO_URL_METRICS_POST_TYPE,
+				'post_content' => $post_content,
+			)
+		);
+
+		$url_metrics = ilo_parse_stored_url_metrics( $post );
+		$this->assertSame( $expected_value, $url_metrics );
+	}
+
+	/**
+	 * Test ilo_store_url_metric().
+	 *
+	 * @covers ::ilo_store_url_metric
+	 */
+	public function test_ilo_store_url_metric() {
+		$url  = home_url( '/' );
+		$slug = ilo_get_url_metrics_slug( array( 'p' => 1 ) );
+
+		$validated_url_metric = array(
+			'viewport' => array(
+				'width'  => 480,
+				'height' => 640,
+			),
+			'elements' => array(
+				array(
+					'isLCP'             => true,
+					'isLCPCandidate'    => true,
+					'xpath'             => '/*[0][self::HTML]/*[1][self::BODY]/*[0][self::DIV]/*[1][self::MAIN]/*[0][self::DIV]/*[0][self::FIGURE]/*[0][self::IMG]',
+					'intersectionRatio' => 1,
+				),
+			),
+		);
+
+		$post_id = ilo_store_url_metric( $url, $slug, $validated_url_metric );
+		$this->assertIsInt( $post_id );
+
+		$post = ilo_get_url_metrics_post( $slug );
+		$this->assertInstanceOf( WP_Post::class, $post );
+		$this->assertSame( $post_id, $post->ID );
+
+		$url_metrics = ilo_parse_stored_url_metrics( $post );
+		$this->assertCount( 1, $url_metrics );
+		$this->assertArrayHasKey( 'timestamp', $url_metrics[0] );
+		$this->assertIsFloat( $url_metrics[0]['timestamp'] );
+		$this->assertLessThanOrEqual( microtime( true ), $url_metrics[0]['timestamp'] );
+
+		$again_post_id = ilo_store_url_metric( $url, $slug, $validated_url_metric );
+		$post          = get_post( $again_post_id );
+		$this->assertSame( $post_id, $again_post_id );
+		$url_metrics = ilo_parse_stored_url_metrics( $post );
+		$this->assertCount( 2, $url_metrics );
+
+		foreach ( $url_metrics as $url_metric ) {
+			$this->assertArrayHasKey( 'timestamp', $url_metric );
+			$this->assertIsFloat( $url_metric['timestamp'] );
+			$this->assertLessThanOrEqual( microtime( true ), $url_metric['timestamp'] );
+			unset( $url_metric['timestamp'] );
+			$this->assertSame( $validated_url_metric, $url_metric );
+		}
+	}
+}
diff --git a/tests/modules/images/image-loading-optimization/storage/rest-api-tests.php b/tests/modules/images/image-loading-optimization/storage/rest-api-tests.php
new file mode 100644
index 000000000..0594a5f2d
--- /dev/null
+++ b/tests/modules/images/image-loading-optimization/storage/rest-api-tests.php
@@ -0,0 +1,258 @@
+<?php
+/**
+ * Tests for image-loading-optimization module storage/rest-api.php.
+ *
+ * @package performance-lab
+ * @group   image-loading-optimization
+ */
+
+class ILO_Storage_REST_API_Tests extends WP_UnitTestCase {
+
+	/**
+	 * @var string
+	 */
+	const ROUTE = '/' . ILO_REST_API_NAMESPACE . ILO_URL_METRICS_ROUTE;
+
+	/**
+	 * Test ilo_register_endpoint().
+	 *
+	 * @covers ::ilo_register_endpoint
+	 */
+	public function test_ilo_register_endpoint_hooked() {
+		$this->assertSame( 10, has_action( 'rest_api_init', 'ilo_register_endpoint' ) );
+	}
+
+	/**
+	 * Test good params.
+	 *
+	 * @covers ::ilo_register_endpoint
+	 * @covers ::ilo_handle_rest_request
+	 */
+	public function test_rest_request_good_params() {
+		$request      = new WP_REST_Request( 'POST', self::ROUTE );
+		$valid_params = $this->get_valid_params();
+		$this->assertCount( 0, get_posts( array( 'post_type' => ILO_URL_METRICS_POST_TYPE ) ) );
+		$request->set_body_params( $valid_params );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertSame( 200, $response->get_status() );
+
+		$data = $response->get_data();
+		$this->assertTrue( $data['success'] );
+
+		$this->assertCount( 1, get_posts( array( 'post_type' => ILO_URL_METRICS_POST_TYPE ) ) );
+		$post = ilo_get_url_metrics_post( $valid_params['slug'] );
+		$this->assertInstanceOf( WP_Post::class, $post );
+
+		$url_metrics = ilo_parse_stored_url_metrics( $post );
+		$this->assertCount( 1, $url_metrics );
+		foreach ( array( 'viewport', 'elements' ) as $key ) {
+			$this->assertSame( $valid_params[ $key ], $url_metrics[0][ $key ] );
+		}
+		$this->assertArrayHasKey( 'timestamp', $url_metrics[0] );
+	}
+
+	/**
+	 * Data provider for test_rest_request_bad_params.
+	 *
+	 * @return array
+	 */
+	public function data_provider_invalid_params(): array {
+		$valid_element = $this->get_valid_params()['elements'][0];
+
+		return array_map(
+			function ( $params ) {
+				return array(
+					'params' => array_merge( $this->get_valid_params(), $params ),
+				);
+			},
+			array(
+				'bad_url'                                  => array(
+					'url' => 'bad://url',
+				),
+				'other_origin_url'                         => array(
+					'url' => 'https://bogus.example.com/',
+				),
+				'bad_slug'                                 => array(
+					'slug' => '<script>document.write("evil")</script>',
+				),
+				'bad_nonce'                                => array(
+					'nonce' => 'not even a hash',
+				),
+				'invalid_nonce'                            => array(
+					'nonce' => ilo_get_url_metrics_storage_nonce( ilo_get_url_metrics_slug( array( 'different' => 'query vars' ) ) ),
+				),
+				'invalid_viewport_type'                    => array(
+					'viewport' => '640x480',
+				),
+				'invalid_viewport_values'                  => array(
+					'viewport' => array(
+						'breadth' => 100,
+						'depth'   => 200,
+					),
+				),
+				'invalid_elements_type'                    => array(
+					'elements' => 'bad',
+				),
+				'invalid_elements_prop_is_lcp'             => array(
+					'elements' => array(
+						array_merge(
+							$valid_element,
+							array(
+								'isLCP' => 'totally!',
+							)
+						),
+					),
+				),
+				'invalid_elements_prop_xpath'              => array(
+					'elements' => array(
+						array_merge(
+							$valid_element,
+							array(
+								'xpath' => 'html > body img',
+							)
+						),
+					),
+				),
+				'invalid_elements_prop_intersection_ratio' => array(
+					'elements' => array(
+						array_merge(
+							$valid_element,
+							array(
+								'intersectionRatio' => - 1,
+							)
+						),
+					),
+				),
+			)
+		);
+	}
+
+	/**
+	 * Test bad params.
+	 *
+	 * @covers ::ilo_register_endpoint
+	 * @covers ::ilo_handle_rest_request
+	 *
+	 * @dataProvider data_provider_invalid_params
+	 */
+	public function test_rest_request_bad_params( array $params ) {
+		$request = new WP_REST_Request( 'POST', self::ROUTE );
+		$request->set_body_params( $params );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertSame( 400, $response->get_status() );
+		$this->assertSame( 'rest_invalid_param', $response->get_data()['code'] );
+	}
+
+	/**
+	 * Test REST API request when metric storage is locked.
+	 *
+	 * @covers ::ilo_register_endpoint
+	 * @covers ::ilo_handle_rest_request
+	 */
+	public function test_rest_request_locked() {
+		ilo_set_url_metric_storage_lock();
+
+		$request = new WP_REST_Request( 'POST', self::ROUTE );
+		$request->set_body_params( $this->get_valid_params() );
+
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertSame( 403, $response->get_status() );
+		$this->assertSame( 'url_metric_storage_locked', $response->get_data()['code'] );
+	}
+
+	/**
+	 * Test sending viewport data that isn't needed for a specific breakpoint.
+	 *
+	 * @covers ::ilo_register_endpoint
+	 * @covers ::ilo_handle_rest_request
+	 */
+	public function test_rest_request_breakpoint_not_needed_for_any_breakpoint() {
+		add_filter( 'ilo_url_metric_storage_lock_ttl', '__return_zero' );
+
+		// First fully populate the sample for all breakpoints.
+		$sample_size     = ilo_get_url_metrics_breakpoint_sample_size();
+		$viewport_widths = array_merge( ilo_get_breakpoint_max_widths(), array( 1000 ) );
+		foreach ( $viewport_widths as $viewport_width ) {
+			for ( $i = 0; $i < $sample_size; $i++ ) {
+				$valid_params                      = $this->get_valid_params();
+				$valid_params['viewport']['width'] = $viewport_width;
+				$request                           = new WP_REST_Request( 'POST', self::ROUTE );
+				$request->set_body_params( $valid_params );
+				$response = rest_get_server()->dispatch( $request );
+				$this->assertSame( 200, $response->get_status() );
+			}
+		}
+
+		// The next request with the same sample size will be rejected.
+		$request = new WP_REST_Request( 'POST', self::ROUTE );
+		$request->set_body_params( $this->get_valid_params() );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertSame( 403, $response->get_status() );
+	}
+
+	/**
+	 * Test sending viewport data that isn't needed for any breakpoint.
+	 *
+	 * @covers ::ilo_register_endpoint
+	 * @covers ::ilo_handle_rest_request
+	 */
+	public function test_rest_request_breakpoint_not_needed_for_specific_breakpoint() {
+		add_filter( 'ilo_url_metric_storage_lock_ttl', '__return_zero' );
+
+		// First fully populate the sample for a given breakpoint.
+		$sample_size = ilo_get_url_metrics_breakpoint_sample_size();
+		for ( $i = 0; $i < $sample_size; $i++ ) {
+			$valid_params                      = $this->get_valid_params();
+			$valid_params['viewport']['width'] = 480;
+			$request                           = new WP_REST_Request( 'POST', self::ROUTE );
+			$request->set_body_params( $valid_params );
+			$response = rest_get_server()->dispatch( $request );
+			$this->assertSame( 200, $response->get_status() );
+		}
+
+		// The next request with the same sample size will be rejected.
+		$request = new WP_REST_Request( 'POST', self::ROUTE );
+		$request->set_body_params( $this->get_valid_params() );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertSame( 403, $response->get_status() );
+	}
+
+	/**
+	 * Gets valid params.
+	 *
+	 * @return array
+	 */
+	private function get_valid_params(): array {
+		$slug = ilo_get_url_metrics_slug( array() );
+		return array_merge(
+			array(
+				'url'   => home_url( '/' ),
+				'slug'  => $slug,
+				'nonce' => ilo_get_url_metrics_storage_nonce( $slug ),
+			),
+			$this->get_sample_validated_url_metric()
+		);
+	}
+
+	/**
+	 * Gets sample validated URL metric data.
+	 *
+	 * @return array
+	 */
+	private function get_sample_validated_url_metric(): array {
+		return array(
+			'viewport' => array(
+				'width'  => 480,
+				'height' => 640,
+			),
+			'elements' => array(
+				array(
+					'isLCP'             => true,
+					'isLCPCandidate'    => true,
+					'xpath'             => '/*[0][self::HTML]/*[1][self::BODY]/*[0][self::DIV]/*[1][self::MAIN]/*[0][self::DIV]/*[0][self::FIGURE]/*[0][self::IMG]',
+					'intersectionRatio' => 1,
+				),
+			),
+		);
+	}
+}
diff --git a/tests/modules/images/webp-uploads/helper-tests.php b/tests/modules/images/webp-uploads/helper-tests.php
index 9fa6fbbd6..5ff31c75a 100644
--- a/tests/modules/images/webp-uploads/helper-tests.php
+++ b/tests/modules/images/webp-uploads/helper-tests.php
@@ -364,6 +364,7 @@ public function it_should_return_empty_array_when_filter_returns_empty_array() {
 	 * @test
 	 */
 	public function it_should_return_default_transforms_when_filter_returns_non_array_type() {
+		/** @phpstan-ignore-next-line */
 		add_filter( 'webp_uploads_upload_image_mime_transforms', '__return_null' );
 
 		$default_transforms = array(