diff --git a/wp-includes/media.php b/wp-includes/media.php
index 30df3226ce..fc69bdcdc8 100644
--- a/wp-includes/media.php
+++ b/wp-includes/media.php
@@ -813,13 +813,14 @@ function wp_get_attachment_image($attachment_id, $size = 'thumbnail', $icon = fa
 
 		// Generate srcset and sizes if not already present.
 		if ( empty( $attr['srcset'] ) ) {
-			$srcset = wp_get_attachment_image_srcset( $attachment_id, $size );
-			$sizes  = wp_get_attachment_image_sizes( $attachment_id, $size, $width );
+			$image_meta = wp_get_attachment_metadata( $attachment_id );
+			$size_array = array( absint( $width ), absint( $height ) );
+			$sources = wp_calculate_image_srcset( $src, $size_array, $image_meta, $attachment_id );
 
-			if ( $srcset && $sizes ) {
-				$attr['srcset'] = $srcset;
+			if ( count( $sources ) > 1 ) {
+				$attr['srcset'] = wp_image_srcset_attr( $sources, $size_array, $image_meta, $attachment_id );
 
-				if ( empty( $attr['sizes'] ) ) {
+				if ( empty( $attr['sizes'] ) && ( $sizes = wp_get_attachment_image_sizes( $size_array, $image_meta, $attachment_id ) ) ) {
 					$attr['sizes'] = $sizes;
 				}
 			}
@@ -864,15 +865,118 @@ function wp_get_attachment_image_url( $attachment_id, $size = 'thumbnail', $icon
 }
 
 /**
- * Retrieves an array of URLs and pixel widths representing sizes of an image.
- *
- * The purpose is to populate a source set when creating responsive image markup.
+ * Private, do not use
+ */
+function _wp_upload_dir_baseurl() {
+	static $baseurl = null;
+
+	if ( ! $baseurl ) {
+		$uploads_dir = wp_upload_dir();
+		$baseurl = $uploads_dir['baseurl'];
+	}
+
+	return $baseurl;
+}
+
+/**
+ * Private, do not use
+ */
+function _wp_get_image_size_from_meta( $size, $image_meta ) {
+	if ( $size === 'full' ) {
+		return array(
+			absint( $image_meta['width'] ),
+			absint( $image_meta['height'] ),
+		);
+	} elseif ( ! empty( $image_meta['sizes'][$size] ) ) {
+		return array(
+			absint( $image_meta['sizes'][$size]['width'] ),
+			absint( $image_meta['sizes'][$size]['height'] ),
+		);
+	}
+
+	return false;
+}
+
+/**
+ * Retrieves the value for an image attachment's 'srcset' attribute.
  *
  * @since 4.4.0
  *
- * @param int          $attachment_id Image attachment ID.
+
+ * @param int          $attachment_id Optional. Image attachment ID.
  * @param array|string $size          Image size. Accepts any valid image size, or an array of width and height
  *                                    values in pixels (in that order). Default 'medium'.
+ * @param array        $image_meta    Optional. The image meta data.
+ * @return string|bool A 'srcset' value string or false.
+ */
+function wp_get_attachment_image_srcset( $attachment_id, $size = 'medium', $image_meta = null ) {
+	if ( ! $image = wp_get_attachment_image_src( $attachment_id, $size ) ) {
+		return false;
+	}
+
+	if ( ! is_array( $image_meta ) ) {
+		$image_meta = get_post_meta( $attachment_id, '_wp_attachment_metadata', true );
+	}
+
+	$image_url = $image[0];
+	$size_array = array(
+		absint( $image[1] ),
+		absint( $image[2] )
+	);
+
+	// Calculate the sources for the srcset.
+	$sources = wp_calculate_image_srcset( $image_url, $size_array, $image_meta, $attachment_id );
+
+	// Only return a srcset value if there is more than one source.
+	if ( count( $sources ) < 2 ) {
+		return false;
+	}
+
+	return wp_image_srcset_attr( $sources, $size_array, $image_meta, $attachment_id );
+}
+
+
+/**
+ * A helper function to concatenate and filter the srcset attribute value.
+ *
+ * @since 4.4.0
+ *
+ * @param array   $sources       The array containing image sizes data as returned by wp_calculate_image_srcset().
+ * @param array   $size_array    Array of width and height values in pixels (in that order).
+ * @param array   $image_meta    The image meta data.
+ * @param int     $attachment_id The image attachment post id to pass to the filter.
+ * @return string The srcset attribute value.
+ */
+function wp_image_srcset_attr( $sources, $size_array, $image_meta, $attachment_id ) {
+	$srcset = '';
+
+	foreach ( $sources as $source ) {
+		$srcset .= $source['url'] . ' ' . $source['value'] . $source['descriptor'] . ', ';
+	}
+
+	/**
+	 * Filter the output of wp_get_attachment_image_srcset().
+	 *
+	 * @since 4.4.0
+	 *
+	 * @param string       $srcset        A source set formatted for a `srcset` attribute.
+	 * @param int          $attachment_id Image attachment ID.
+	 * @param array|string $size          Image size. Accepts any valid image size, or an array of width and height
+	 *                                    values in pixels (in that order). Default 'medium'.
+	 * @param array        $image_meta    The image meta data.
+	 */
+	return apply_filters( 'wp_get_attachment_image_srcset', rtrim( $srcset, ', ' ), $attachment_id, $size_array, $image_meta );
+}
+
+/**
+ * A helper function to caclulate the image sources to include in a srcset attribute.
+ *
+ * @since 4.4.0
+ *
+ * @param string $image_name    The file name, path, URL or partial path or URL of the image being matched.
+ * @param array  $size_array    Array of width and height values in pixels (in that order).
+ * @param array  $image_meta    The image meta data.
+ * @param int    $attachment_id Optional. The image attachment post id to pass to the filter.
  * @return array|bool $sources {
  *     Array image candidate values containing a URL, descriptor type, and
  *     descriptor value. False if none exist.
@@ -886,103 +990,91 @@ function wp_get_attachment_image_url( $attachment_id, $size = 'thumbnail', $icon
  * }
  *
  */
-function wp_get_attachment_image_srcset_array( $attachment_id, $size = 'medium' ) {
-	// Get the intermediate size.
-	$image = image_get_intermediate_size( $attachment_id, $size );
-
-	// Get the post meta.
-	$img_meta = wp_get_attachment_metadata( $attachment_id );
-	if ( ! is_array( $img_meta ) ) {
+function wp_calculate_image_srcset( $image_name, $size_array, $image_meta, $attachment_id = 0 ) {
+	if ( empty( $image_meta['sizes'] ) ) {
 		return false;
 	}
 
-	// Extract the height and width from the intermediate or the full size.
-	$img_width  = ( $image ) ? $image['width']  : $img_meta['width'];
-	$img_height = ( $image ) ? $image['height'] : $img_meta['height'];
+	$image_sizes = $image_meta['sizes'];
 
-	// Bail early if the width isn't greater than zero.
-	if ( ! $img_width > 0 ) {
+	// Get the height and width for the image.
+	$image_width = (int) $size_array[0];
+	$image_height = (int) $size_array[1];
+
+	// Bail early if error/no width.
+	if ( $image_width < 1 ) {
 		return false;
 	}
 
-	// Use the URL from the intermediate size or build the url from the metadata.
-	if ( ! empty( $image['url'] ) ) {
-		$img_url = $image['url'];
-	} else {
-		$uploads_dir = wp_upload_dir();
-		$img_file = ( $image ) ? path_join( dirname( $img_meta['file'] ) , $image['file'] ) : $img_meta['file'];
-		$img_url = $uploads_dir['baseurl'] . '/' . $img_file;
-	}
-
-	$img_sizes = $img_meta['sizes'];
-
 	// Add full size to the img_sizes array.
-	$img_sizes['full'] = array(
-		'width'  => $img_meta['width'],
-		'height' => $img_meta['height'],
-		'file'   => wp_basename( $img_meta['file'] )
+	$image_sizes['full'] = array(
+		'width'  => $image_meta['width'],
+		'height' => $image_meta['height'],
+		'file'   => wp_basename( $image_meta['file'] ),
 	);
 
+	$image_baseurl = _wp_upload_dir_baseurl();
+	$dirname = dirname( $image_meta['file'] );
+
+	if ( $dirname !== '.' ) {
+		$image_baseurl = path_join( $image_baseurl, $dirname );
+	}
+
 	// Calculate the image aspect ratio.
-	$img_ratio = $img_height / $img_width;
+	$image_ratio = $image_height / $image_width;
 
 	/*
 	 * Images that have been edited in WordPress after being uploaded will
 	 * contain a unique hash. Look for that hash and use it later to filter
 	 * out images that are leftovers from previous versions.
 	 */
-	$img_edited = preg_match( '/-e[0-9]{13}/', $img_url, $img_edit_hash );
+	$image_edited = preg_match( '/-e[0-9]{13}/', $image_name, $image_edit_hash );
 
 	/**
-	 * Filter the maximum width included in a srcset attribute.
+	 * Filter the maximum width included in a 'srcset' attribute.
 	 *
 	 * @since 4.4.0
 	 *
-	 * @param array|string $size Size of image, either array or string.
+	 * @param int          $max_width  The maximum width to include in the 'srcset'. Default '1600'.
+	 * @param array|string $size_array Array of width and height values in pixels (in that order).
 	 */
-	$max_srcset_width = apply_filters( 'max_srcset_image_width', 1600, $size );
+	$max_srcset_width = apply_filters( 'max_srcset_image_width', 1600, $size_array );
+
+	// Array to hold URL candidates.
+	$sources = array();
 
 	/*
-	 * Set up arrays to hold url candidates and matched image sources so
-	 * we can avoid duplicates without looping through the full sources array
+	 * Loop through available images. Only use images that are resized
+	 * versions of the same edit.
 	 */
-	$candidates = $sources = array();
+	foreach ( $image_sizes as $image ) {
 
-	/*
-	 * Loop through available images and only use images that are resized
-	 * versions of the same rendition.
-	 */
-	foreach ( $img_sizes as $img ) {
-
-		// Filter out images that are leftovers from previous renditions.
-		if ( $img_edited && ! strpos( $img['file'], $img_edit_hash[0] ) ) {
+		// Filter out images that are from previous edits.
+		if ( $image_edited && ! strpos( $image['file'], $image_edit_hash[0] ) ) {
 			continue;
 		}
 
 		// Filter out images that are wider than $max_srcset_width.
-		if ( $max_srcset_width && $img['width'] > $max_srcset_width ) {
+		if ( $max_srcset_width && $image['width'] > $max_srcset_width ) {
 			continue;
 		}
 
-		$candidate_url = path_join( dirname( $img_url ), $img['file'] );
+		$candidate_url = $image['file'];
 
 		// Calculate the new image ratio.
-		if ( $img['width'] ) {
-			$img_ratio_compare = $img['height'] / $img['width'];
+		if ( $image['width'] ) {
+			$image_ratio_compare = $image['height'] / $image['width'];
 		} else {
-			$img_ratio_compare = 0;
+			$image_ratio_compare = 0;
 		}
 
 		// If the new ratio differs by less than 0.01, use it.
-		if ( abs( $img_ratio - $img_ratio_compare ) < 0.01 && ! in_array( $candidate_url, $candidates ) ) {
-			// Add the URL to our list of candidates.
-			$candidates[] = $candidate_url;
-
-			// Add the url, descriptor, and value to the sources array to be returned.
-			$sources[] = array(
-				'url'        => $candidate_url,
+		if ( abs( $image_ratio - $image_ratio_compare ) < 0.01 && ! array_key_exists( $candidate_url, $sources ) ) {
+			// Add the URL, descriptor, and value to the sources array to be returned.
+			$sources[ $image['width'] ] = array(
+				'url'        => path_join( $image_baseurl, $candidate_url ),
 				'descriptor' => 'w',
-				'value'      => $img['width'],
+				'value'      => $image['width'],
 			);
 		}
 	}
@@ -992,81 +1084,55 @@ function wp_get_attachment_image_srcset_array( $attachment_id, $size = 'medium'
 	 *
 	 * @since 4.4.0
 	 *
-	 * @param array        $sources       An array of image urls and widths.
+	 * @param array        $sources       An array of image URLs and widths.
 	 * @param int          $attachment_id Attachment ID for image.
 	 * @param array|string $size          Image size. Accepts any valid image size, or an array of width and height
 	 *                                    values in pixels (in that order). Default 'medium'.
+	 * @param array        $image_meta    The image meta data.
+
 	 */
-	return apply_filters( 'wp_get_attachment_image_srcset_array', $sources, $attachment_id, $size );
+	return apply_filters( 'wp_get_attachment_image_srcset_array', array_values( $sources ), $attachment_id, $size_array, $image_meta );
 }
 
 /**
- * Retrieves the value for an image attachment's 'srcset' attribute.
+ * Create `sizes` attribute value for an image.
  *
  * @since 4.4.0
  *
- * @param int          $attachment_id Image attachment ID.
- * @param array|string $size          Image size. Accepts any valid image size, or an array of width and height
- *                                    values in pixels (in that order). Default 'medium'.
- * @return string|bool A 'srcset' value string or false.
- */
-function wp_get_attachment_image_srcset( $attachment_id, $size = 'medium' ) {
-	$srcset_array = wp_get_attachment_image_srcset_array( $attachment_id, $size );
-
-	// Only return a srcset value if there is more than one source.
-	if ( count( $srcset_array ) <= 1 ) {
-		return false;
-	}
-
-	$srcset = '';
-	foreach ( $srcset_array as $source ) {
-		$srcset .= $source['url'] . ' ' . $source['value'] . $source['descriptor'] . ', ';
-	}
-
-	/**
-	 * Filter the output of wp_get_attachment_image_srcset().
-	 *
-	 * @since 4.4.0
-	 *
-	 * @param string       $srcset        A source set formated for a `srcset` attribute.
-	 * @param int          $attachment_id Attachment ID for image.
-	 * @param array|string $size          Image size. Accepts any valid image size, or an array of width and height
-	 *                                    values in pixels (in that order). Default 'medium'.
-	 */
-	return apply_filters( 'wp_get_attachment_image_srcset', rtrim( $srcset, ', ' ), $attachment_id, $size );
-}
-
-/**
- * Retrieves a source size attribute for an image from an array of values.
+ * @param array|string $size          Image size. Accepts any valid image size name (thumbnail, medium, etc.),
+ *                                    or an array of width and height values in pixels (in that order).
+ * @param array        $image_meta    Optional. The image meta data.
+ * @param int          $attachment_id Optional. Image attachment ID. Either $image_meta or $attachment_id is needed
+ *                                    when using image size name.
  *
- * @since 4.4.0
- *
- * @param int          $attachment_id Image attachment ID.
- * @param array|string $size          Image size. Accepts any valid image size, or an array of width and height
- *                                    values in pixels (in that order). Default 'medium'.
- * @param int          $width         Optional. Display width of the image.
  * @return string|bool A valid source size value for use in a 'sizes' attribute or false.
  */
-function wp_get_attachment_image_sizes( $attachment_id, $size = 'medium', $width = null ) {
-	// Try to get the image width from the $width parameter.
-	if ( is_numeric( $width ) ) {
-		$img_width = (int) $width;
-	// Next, see if a width value was passed in the $size parameter.
+function wp_get_attachment_image_sizes( $size, $image_meta = null, $attachment_id = 0 ) {
+	$width = 0;
+
+	if ( is_numeric( $size ) ) {
+		$width = absint( $size );
 	} elseif ( is_array( $size ) ) {
-		$img_width = $size[0];
-	// Finally, use the $size name to return the width of the image.
-	} else {
-		$image = image_get_intermediate_size( $attachment_id, $size );
-		$img_width = $image ? $image['width'] : false;
+		$width = absint( $size[0] );
+	} elseif ( is_string( $size ) ) {
+		if ( ! $image_meta && $attachment_id ) {
+			$image_meta = wp_get_attachment_metadata( $attachment_id );
+		}
+
+		if ( $image_meta ) {
+			$width = _wp_get_image_size_from_meta( $size, $image_meta );
+			if ( $width ) {
+				$width = $width[0];
+			}
+		}
 	}
 
-	// Bail early if $img_width isn't set.
-	if ( ! $img_width ) {
+	if ( ! $width ) {
 		return false;
 	}
 
 	// Setup the default sizes attribute.
-	$sizes = sprintf( '(max-width: %1$dpx) 100vw, %1$dpx', $img_width );
+	$sizes = sprintf( '(max-width: %1$dpx) 100vw, %1$dpx', (int) $width );
 
 	/**
 	 * Filter the output of wp_get_attachment_image_sizes().
@@ -1074,12 +1140,12 @@ function wp_get_attachment_image_sizes( $attachment_id, $size = 'medium', $width
 	 * @since 4.4.0
 	 *
 	 * @param string       $sizes         A source size value for use in a 'sizes' attribute.
-	 * @param int          $attachment_id Post ID of the original image.
 	 * @param array|string $size          Image size. Accepts any valid image size, or an array of width and height
 	 *                                    values in pixels (in that order). Default 'medium'.
-	 * @param int          $width         Display width of the image.
+	 * @param array        $image_meta    The image meta data.
+	 * @param int          $attachment_id Post ID of the original image.
 	 */
-	return apply_filters( 'wp_get_attachment_image_sizes', $sizes, $attachment_id, $size, $width );
+	return apply_filters( 'wp_get_attachment_image_sizes', $sizes, $size, $image_meta, $attachment_id );
 }
 
 /**
@@ -1087,7 +1153,7 @@ function wp_get_attachment_image_sizes( $attachment_id, $size = 'medium', $width
  *
  * @since 4.4.0
  *
- * @see wp_img_add_srcset_and_sizes()
+ * @see wp_image_add_srcset_and_sizes()
  *
  * @param string $content The raw post content to be filtered.
  * @return string Converted content with 'srcset' and 'sizes' attributes added to images.
@@ -1095,29 +1161,33 @@ function wp_get_attachment_image_sizes( $attachment_id, $size = 'medium', $width
 function wp_make_content_images_responsive( $content ) {
 	$images = get_media_embedded_in_content( $content, 'img' );
 
-	$attachment_ids = array();
+	$selected_images = $attachment_ids = array();
 
 	foreach( $images as $image ) {
-		if ( preg_match( '/wp-image-([0-9]+)/i', $image, $class_id ) ) {
-			$attachment_id = (int) $class_id[1];
-			if ( $attachment_id ) {
-				$attachment_ids[] = $attachment_id;
-			}
+		if ( false === strpos( $image, ' srcset="' ) && preg_match( '/wp-image-([0-9]+)/i', $image, $class_id ) &&
+			( $attachment_id = absint( $class_id[1] ) ) ) {
+
+			// If exactly the same image tag is used more than once, overwrite it.
+			// All identical tags will be replaced later with str_replace().
+			$selected_images[ $image ] = $attachment_id;
+			// Overwrite the ID when the same image is included more than once.
+			$attachment_ids[ $attachment_id ] = true;
 		}
 	}
 
-	if ( 0 < count( $attachment_ids ) ) {
+	if ( count( $attachment_ids ) > 1 ) {
 		/*
-		 * Warm object caches for use with wp_get_attachment_metadata.
+		 * Warm object cache for use with get_post_meta().
 		 *
 		 * To avoid making a database call for each image, a single query
 		 * warms the object cache with the meta information for all images.
 		 */
-		_prime_post_caches( $attachment_ids, false, true );
+		update_meta_cache( 'post', array_keys( $attachment_ids ) );
 	}
 
-	foreach( $images as $image ) {
-		$content = str_replace( $image, wp_img_add_srcset_and_sizes( $image ), $content );
+	foreach ( $selected_images as $image => $attachment_id ) {
+		$image_meta = get_post_meta( $attachment_id, '_wp_attachment_metadata', true );
+		$content = str_replace( $image, wp_image_add_srcset_and_sizes( $image, $image_meta, $attachment_id ), $content );
 	}
 
 	return $content;
@@ -1131,74 +1201,76 @@ function wp_make_content_images_responsive( $content ) {
  * @see wp_get_attachment_image_srcset()
  * @see wp_get_attachment_image_sizes()
  *
- * @param string $image An HTML 'img' element to be filtered.
+ * @param string $image         An HTML 'img' element to be filtered.
+ * @param array  $image_meta    The image meta data.
+ * @param int    $attachment_id Image attachment ID.
  * @return string Converted 'img' element with `srcset` and `sizes` attributes added.
  */
-function wp_img_add_srcset_and_sizes( $image ) {
-	// Return early if a 'srcset' attribute already exists.
-	if ( false !== strpos( $image, ' srcset="' ) ) {
+function wp_image_add_srcset_and_sizes( $image, $image_meta, $attachment_id ) {
+	// Ensure the image meta exists
+	if ( empty( $image_meta['sizes'] ) ) {
 		return $image;
 	}
 
-	// Parse id, size, width, and height from the `img` element.
-	$id     = preg_match( '/wp-image-([0-9]+)/i', $image, $match_id     ) ? (int) $match_id[1]     : false;
-	$size   = preg_match( '/size-([^\s|"]+)/i',   $image, $match_size   ) ? $match_size[1]         : false;
-	$width  = preg_match( '/ width="([0-9]+)"/',  $image, $match_width  ) ? (int) $match_width[1]  : false;
+	$src = preg_match( '/src="([^"]+)"/', $image, $match_src ) ? $match_src[1] : '';
+	list( $src ) = explode( '?', $src );
 
-	if ( $id && ! $size ) {
-		$height = preg_match( '/ height="([0-9]+)"/', $image, $match_height ) ? (int) $match_height[1] : false;
-
-		if ( $width && $height ) {
-			$size = array( $width, $height );
-		}
+	// Return early if we coudn't get the image source.
+	if ( ! $src ) {
+		return $image;
 	}
 
-	/*
-	 * If attempts to parse the size value failed, attempt to use the image
-	 * metadata to match the 'src' against the available sizes for an attachment.
-	 */
-	if ( $id && ! $size ) {
-		$meta = wp_get_attachment_metadata( $id );
+	// Bail early when an image has been inserted and later edited.
+	if ( preg_match( '/-e[0-9]{13}/', $image_meta['file'], $img_edit_hash ) &&
+		strpos( wp_basename( $src ), $img_edit_hash[0] ) === false ) {
 
-		// Parse the image src value from the img element.
-		$src = preg_match( '/src="([^"]+)"/', $image, $match_src ) ? $match_src[1] : false;
+		return $image;
+	}
 
-		// Return early if the metadata does not exist or the src value is empty.
-		if ( ! $meta || ! $src ) {
-			return $image;
-		}
+	$width  = preg_match( '/ width="([0-9]+)"/',  $image, $match_width  ) ? (int) $match_width[1]  : 0;
+	$height = preg_match( '/ height="([0-9]+)"/', $image, $match_height ) ? (int) $match_height[1] : 0;
 
+	if ( ! $width || ! $height ) {
 		/*
-		 * First, see if the file is the full size image. If not, loop through
-		 * the intermediate sizes until we find a file that matches.
+		 * If attempts to parse the size value failed, attempt to use the image
+		 * metadata to match the image file name from 'src' against the available sizes for an attachment.
 		 */
 		$image_filename = wp_basename( $src );
 
-		if ( $image_filename === basename( $meta['file'] ) ) {
-			$size = 'full';
+		if ( $image_filename === wp_basename( $image_meta['file'] ) ) {
+			$width = (int) $image_meta['width'];
+			$height = (int) $image_meta['height'];
 		} else {
-			foreach( $meta['sizes'] as $image_size => $image_size_data ) {
+			foreach( $image_meta['sizes'] as $image_size_data ) {
 				if ( $image_filename === $image_size_data['file'] ) {
-					$size = $image_size;
+					$width = (int) $image_size_data['width'];
+					$height = (int) $image_size_data['height'];
 					break;
 				}
 			}
 		}
-
 	}
 
-	// If ID and size exist, try for 'srcset' and 'sizes' and update the markup.
-	if ( $id && $size ) {
-		$srcset = wp_get_attachment_image_srcset( $id, $size );
-		$sizes  = wp_get_attachment_image_sizes( $id, $size, $width );
+	if ( ! $width || ! $height ) {
+		return $image;
+	}
 
-		if ( $srcset && $sizes ) {
-			// Format the srcset and sizes string and escape attributes.
-			$srcset_and_sizes = sprintf( ' srcset="%s" sizes="%s"', esc_attr( $srcset ), esc_attr( $sizes) );
+	$size_array = array( $width, $height );
+	$sources = wp_calculate_image_srcset( $src, $size_array, $image_meta, $attachment_id );
 
-			// Add srcset and sizes attributes to the image markup.
-			$image = preg_replace( '/<img ([^>]+)[\s?][\/?]>/', '<img $1' . $srcset_and_sizes . ' />', $image );
-		}
+	$srcset = $sizes = '';
+	// Only calculate srcset and sizes values if there is more than one source.
+	if ( count( $sources ) > 1 ) {
+		$srcset = wp_image_srcset_attr( $sources, $size_array, $image_meta, $attachment_id );
+		$sizes = wp_get_attachment_image_sizes( $size_array, $image_meta, $attachment_id );
+	}
+
+	if ( $srcset && $sizes ) {
+		// Format the srcset and sizes string and escape attributes.
+		$srcset_and_sizes = sprintf( ' srcset="%s" sizes="%s"', esc_attr( $srcset ), esc_attr( $sizes ) );
+
+		// Add srcset and sizes attributes to the image markup.
+		$image = preg_replace( '/<img ([^>]+?)[\/ ]*>/', '<img $1' . $srcset_and_sizes . ' />', $image );
 	}
 
 	return $image;
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 4bdcdd1248..cd88b8f650 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.4-beta1-35411';
+$wp_version = '4.4-beta1-35412';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.