function test_wp_make_content_images_responsive_with_preexisting_srcset() {
    // Generate HTML and add a dummy srcset attribute.
    $image_html = get_image_tag( self::$large_id, '', '', '', 'medium' );
    $image_html = preg_replace( '|<img ([^>]+) />|', '<img $1 ' . 'srcset="image2x.jpg 2x" />', $image_html );

    // The content filter should return the image unchanged.
    $this->assertSame( $image_html, wp_make_content_images_responsive( $image_html ) );
  }