Opt-in to amp-img-auto-sizes experiment when there are converted images

includes/sanitizers/class-amp-img-sanitizer.php

@@ -107,6 +107,21 @@ public function sanitize() {
 
 		$this->determine_dimensions( $need_dimensions );
 		$this->adjust_and_replace_nodes_in_array_map( $need_dimensions );
+
+		/*
+		 * Opt-in to amp-img-auto-sizes experiment.
+		 * This is needed because the sizes attribute is removed from all img elements converted to amp-img
+		 * in order to prevent the undesirable setting of the width. This $meta tag can be removed once the
+		 * experiment ends (and the feature has been fully launched).
+		 * See <https://github.com/ampproject/amphtml/issues/21371> and <https://github.com/ampproject/amp-wp/pull/2036>.
+		 */
+		$head = $this->dom->getElementsByTagName( 'head' )->item( 0 );
+		if ( $head ) {
+			$meta = $this->dom->createElement( 'meta' );
+			$meta->setAttribute( 'name', 'amp-experiments-opt-in' );
+			$meta->setAttribute( 'content', 'amp-img-auto-sizes' );
+			$head->insertBefore( $meta, $head->firstChild );
+		}
 	}
 
 	/**

tests/test-amp-img-sanitizer.php

@@ -239,10 +239,14 @@ public function test_converter( $source, $expected = null ) {
 			$expected = $source;
 		}
 		$dom       = AMP_DOM_Utils::get_dom_from_content( $source );
+		$img_count = $dom->getElementsByTagName( 'img' )->length;
 		$sanitizer = new AMP_Img_Sanitizer( $dom );
 		$sanitizer->sanitize();
 		$content = AMP_DOM_Utils::get_content_from_dom( $dom );
 		$this->assertEquals( $expected, $content );
+
+		$xpath = new DOMXPath( $dom );
+		$this->assertEquals( $img_count ? 1 : 0, $xpath->query( '/html/head/meta[ @name = "amp-experiments-opt-in" ][ @content = "amp-img-auto-sizes" ]' )->length );
 	}
 
 	/**