diff --git a/src/wp-includes/kses.php b/src/wp-includes/kses.php
index a9e8bbdd3ec5f..0ef5803ebc7b4 100644
--- a/src/wp-includes/kses.php
+++ b/src/wp-includes/kses.php
@@ -988,6 +988,9 @@ function wp_kses_split( $content, $allowed_html, $allowed_protocols ) {
 		(<!--.*?(-->|$))   #  - Normative HTML comments.
 		|
 		</[^a-zA-Z][^>]*>  #  - Closing tags with invalid tag names.
+		|
+		<![^>]*>           #  - Invalid markup declaration nodes. Not all invalid nodes
+		                   #    are matched so as to avoid breaking legacy behaviors.
 	)
 	|
 	(<[^>]*(>|$)|>)        # Tag-like spans of text.
@@ -1114,22 +1117,30 @@ function wp_kses_split2( $content, $allowed_html, $allowed_protocols ) {
 	}
 
 	/*
-	 * When a closing tag appears with a name that isn't a valid tag name,
-	 * it must be interpreted as an HTML comment. It extends until the
-	 * first `>` character after the initial opening `</`.
+	 * When certain invalid syntax constructs appear, the HTML parser
+	 * shifts into what's called the "bogus comment state." This is a
+	 * plaintext state that consumes everything until the nearest `>`
+	 * and then transforms the entire span into an HTML comment.
 	 *
 	 * Preserve these comments and do not treat them like tags.
+	 *
+	 * @see https://html.spec.whatwg.org/#bogus-comment-state
 	 */
-	if ( 1 === preg_match( '~^</[^a-zA-Z][^>]*>$~', $content ) ) {
-		$content     = substr( $content, 2, -1 );
-		$transformed = null;
+	if ( 1 === preg_match( '~^(?:</[^a-zA-Z][^>]*>|<![a-z][^>]*>)$~', $content ) ) {
+		/**
+		 * Since the pattern matches `</…>` and also `<!…>`, this will
+		 * preserve the type of the cleaned-up token in the output.
+		 */
+		$opener  = $content[1];
+		$content = substr( $content, 2, -1 );
 
-		while ( $transformed !== $content ) {
-			$transformed = wp_kses( $content, $allowed_html, $allowed_protocols );
-			$content     = $transformed;
-		}
+		do {
+			$prev    = $content;
+			$content = wp_kses( $content, $allowed_html, $allowed_protocols );
+		} while ( $prev !== $content );
 
-		return "</{$transformed}>";
+		// Recombine the modified inner content with the original token structure.
+		return "<{$opener}{$content}>";
 	}
 
 	/*
diff --git a/tests/phpunit/tests/kses.php b/tests/phpunit/tests/kses.php
index 36bf2baf123d3..ea65a89092c07 100644
--- a/tests/phpunit/tests/kses.php
+++ b/tests/phpunit/tests/kses.php
@@ -1936,11 +1936,13 @@ public function filter_wp_kses_object_added_in_html_filter( $tags, $context ) {
 	 *
 	 * @ticket 61009
 	 *
+	 * @dataProvider data_html_containing_various_kinds_of_html_comments
+	 *
 	 * @param string $html_comment    HTML containing a comment; must not be a valid comment
 	 *                                but must be syntax which a browser interprets as a comment.
 	 * @param string $expected_output How `wp_kses()` ought to transform the comment.
 	 */
-	public function wp_kses_preserves_html_comments( $html_comment, $expected_output ) {
+	public function test_wp_kses_preserves_html_comments( $html_comment, $expected_output ) {
 		$this->assertSame(
 			$expected_output,
 			wp_kses( $html_comment, array() ),
@@ -1957,6 +1959,7 @@ public static function data_html_containing_various_kinds_of_html_comments() {
 		return array(
 			'Normative HTML comment'            => array( 'before<!-- this is a comment -->after', 'before<!-- this is a comment -->after' ),
 			'Closing tag with invalid tag name' => array( 'before<//not a tag>after', 'before<//not a tag>after' ),
+			'Incorrectly opened comment (Markup declaration)' => array( 'before<!also not a tag>after', 'before<!also not a tag>after' ),
 		);
 	}