Avoid preloading background images larger than 2MB

plugins/image-prioritizer/helper.php

@@ -234,7 +234,30 @@ static function ( $host ) {
 		);
 	}
 
-	// TODO: Check for the Content-Length and return invalid if it is gigantic?
+	/*
+	 * Validate that the Content-Length is not too massive, as it would be better to err on the side of
+	 * not preloading something so weighty in case the image won't actually end up as LCP.
+	 * The value of 2MB is chosen because according to Web Almanac 2022, the largest image by byte size
+	 * on a page is 1MB at the 90th percentile: <https://almanac.httparchive.org/en/2022/media#fig-12>.
+	 * The 2MB value is double this 1MB size.
+	 */
+	$content_length = (array) wp_remote_retrieve_header( $r, 'content-length' );
+	if ( ! is_numeric( $content_length[0] ) ) {
+		return new WP_Error(
+			'background_image_content_length_unknown',
+			__( 'HEAD request for background image URL did not include a Content-Length response header.', 'image-prioritizer' )
+		);
+	} elseif ( (int) $content_length[0] > 2 * MB_IN_BYTES ) {
+		return new WP_Error(
+			'background_image_content_length_too_large',
+			sprintf(
+				/* translators: %s is the content length of the response  */
+				__( 'HEAD request for background image URL returned Content-Length greater than 2MB: %s.', 'image-prioritizer' ),
+				$content_length[0]
+			)
+		);
+	}
+
 	return true;
 }
 

plugins/image-prioritizer/tests/test-helper.php

@@ -639,6 +639,37 @@ static function ( $pre, $parsed_args, $url ) use ( $video_url ) {
 				'expect_error' => 'background_image_response_not_image',
 			),
 
+			'bad_content_length'          => array(
+				'set_up'       => static function (): string {
+					$image_url = home_url( '/massive-image.jpg' );
+
+					add_filter(
+						'pre_http_request',
+						static function ( $pre, $parsed_args, $url ) use ( $image_url ) {
+							if ( 'HEAD' !== $parsed_args['method'] || $image_url !== $url ) {
+								return $pre;
+							}
+							return array(
+								'headers'  => array(
+									'content-type'   => 'image/jpeg',
+									'content-length' => (string) ( 2 * MB_IN_BYTES + 1 ),
+								),
+								'body'     => '',
+								'response' => array(
+									'code'    => 200,
+									'message' => 'OK',
+								),
+							);
+						},
+						10,
+						3
+					);
+
+					return $image_url;
+				},
+				'expect_error' => 'background_image_content_length_too_large',
+			),
+
 			'bad_redirect'                => array(
 				'set_up'       => static function (): string {
 					$redirect_url = home_url( '/redirect.jpg' );