Accessibility: Improve the image and gallery widgets preview accessibility.

In an authoring context, the image `alt` attribute purpose is different from the one for the front end. For example, screen reader users need to know what the selected image is, even when the original `alt` value is empty.

This change introduces a new pattern for the `alt` text in an authoring context:
- uses the `alt` text if not empty
- when there's no `alt` text, informs users the image has no alternative text and provides a reference to the image filename

Also, makes the gallery media widget preview an unordered list to make screen readers announce the number of images automatically.

Fixes #43137.


git-svn-id: https://develop.svn.wordpress.org/trunk@44767 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/widgets/class-wp-widget-media-gallery.php

@@ -175,34 +175,57 @@ class WP_Widget_Media_Gallery extends WP_Widget_Media {
 		parent::render_control_template_scripts();
 		?>
 		<script type="text/html" id="tmpl-wp-media-widget-gallery-preview">
-			<# var describedById = 'describedBy-' + String( Math.random() ); #>
 			<#
 			var ids = _.filter( data.ids, function( id ) {
 				return ( id in data.attachments );
 			} );
 			#>
 			<# if ( ids.length ) { #>
-				<div class="gallery media-widget-gallery-preview">
+				<ul class="gallery media-widget-gallery-preview" role="list">
 					<# _.each( ids, function( id, index ) { #>
 						<# var attachment = data.attachments[ id ]; #>
 						<# if ( index < 6 ) { #>
-							<dl class="gallery-item">
-								<dt class="gallery-icon">
-								<# if ( attachment.sizes.thumbnail ) { #>
-									<img src="{{ attachment.sizes.thumbnail.url }}" width="{{ attachment.sizes.thumbnail.width }}" height="{{ attachment.sizes.thumbnail.height }}" alt="" />
-								<# } else { #>
-									<img src="{{ attachment.url }}" alt="" />
-								<# } #>
-								<# if ( index === 5 && ids.length > 6 ) { #>
+							<li class="gallery-item">
+								<div class="gallery-icon">
+									<img alt="{{ attachment.alt }}"
+										<# if ( index === 5 && data.ids.length > 6 ) { #> aria-hidden="true" <# } #>
+										<# if ( attachment.sizes.thumbnail ) { #>
+											src="{{ attachment.sizes.thumbnail.url }}" width="{{ attachment.sizes.thumbnail.width }}" height="{{ attachment.sizes.thumbnail.height }}"
+										<# } else { #>
+											src="{{ attachment.url }}"
+										<# } #>
+										<# if ( ! attachment.alt && attachment.filename ) { #>
+											aria-label="
+											<?php
+											echo esc_attr(
+												sprintf(
+													/* translators: %s: the image file name. */
+													__( 'The current image has no alternative text. The file name is: %s' ),
+													'{{ attachment.filename }}'
+												)
+											);
+											?>
+											"
+										<# } #>
+									/>
+									<# if ( index === 5 && data.ids.length > 6 ) { #>
 									<div class="gallery-icon-placeholder">
-										<p class="gallery-icon-placeholder-text">+{{ ids.length - 5 }}</p>
+										<p class="gallery-icon-placeholder-text" aria-label="
+										<?php
+											printf(
+												/* translators: %s: the amount of additional, not visible images in the gallery widget preview. */
+												__( 'Additional images added to this gallery: %s' ),
+												'{{ data.ids.length - 5 }}'
+											);
+										?>
+										">+{{ data.ids.length - 5 }}</p>
 									</div>
-								<# } #>
-								</dt>
-							</dl>
+									<# } #>
+								</div>
+							</li>
 						<# } #>
 					<# } ); #>
-				</div>
+				</ul>
 			<# } else { #>
 				<div class="attachment-media-view">
 					<p class="placeholder"><?php echo esc_html( $this->l10n['no_media_selected'] ); ?></p>

src/wp-includes/widgets/class-wp-widget-media-image.php

@@ -339,7 +339,6 @@ class WP_Widget_Media_Image extends WP_Widget_Media {
 			<# } #>
 		</script>
 		<script type="text/html" id="tmpl-wp-media-widget-image-preview">
-			<# var describedById = 'describedBy-' + String( Math.random() ); #>
 			<# if ( data.error && 'missing_attachment' === data.error ) { #>
 				<div class="notice notice-error notice-alt notice-missing-attachment">
 					<p><?php echo $this->l10n['missing_attachment']; ?></p>
@@ -349,15 +348,21 @@ class WP_Widget_Media_Image extends WP_Widget_Media {
 					<p><?php _e( 'Unable to preview media due to an unknown error.' ); ?></p>
 				</div>
 			<# } else if ( data.url ) { #>
-				<img class="attachment-thumb" src="{{ data.url }}" draggable="false" alt="{{ data.alt }}" <# if ( ! data.alt && data.currentFilename ) { #> aria-describedby="{{ describedById }}" <# } #> />
-				<# if ( ! data.alt && data.currentFilename ) { #>
-					<p class="hidden" id="{{ describedById }}">
-					<?php
-						/* translators: %s: image filename */
-						echo sprintf( __( 'Current image: %s' ), '{{ data.currentFilename }}' );
-					?>
-					</p>
-				<# } #>
+				<img class="attachment-thumb" src="{{ data.url }}" draggable="false" alt="{{ data.alt }}"
+					<# if ( ! data.alt && data.currentFilename ) { #>
+						aria-label="
+						<?php
+						echo esc_attr(
+							sprintf(
+								/* translators: %s: the image file name. */
+								__( 'The current image has no alternative text. The file name is: %s' ),
+								'{{ data.currentFilename }}'
+							)
+						);
+						?>
+						"
+					<# } #>
+				/>
 			<# } #>
 		</script>
 		<?php