Accessibility: change the close "X" in the Media "Attach to existing content" modal in a button.

Props joedolson.

Fixes #36554.

git-svn-id: https://develop.svn.wordpress.org/trunk@37231 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/css/media.css

@@ -302,18 +302,32 @@
 #find-posts-close {
 	width: 36px;
 	height: 36px;
+	border: none;
+	padding: 0;
 	position: absolute;
 	top: 0;
 	right: 0;
 	cursor: pointer;
 	text-align: center;
+	background: none;
 	color: #666;
 }
 
-#find-posts-close:hover {
+#find-posts-close:hover,
+#find-posts-close:focus {
 	color: #00a0d2;
 }
 
+#find-posts-close:focus {
+	outline: none;
+	-webkit-box-shadow:
+		0 0 0 1px #5b9dd9,
+		0 0 2px 1px rgba(30, 140, 190, .8);
+	box-shadow:
+		0 0 0 1px #5b9dd9,
+		0 0 2px 1px rgba(30, 140, 190, .8);
+}
+
 #find-posts-close:before {
 	font: normal 20px/36px dashicons;
 	vertical-align: top;

src/wp-admin/includes/template.php

@@ -1486,7 +1486,7 @@ function find_posts_div($found_action = '') {
 	<div id="find-posts" class="find-box" style="display: none;">
 		<div id="find-posts-head" class="find-box-head">
 			<?php _e( 'Attach to existing content' ); ?>
-			<div id="find-posts-close"></div>
+			<button type="button" id="find-posts-close"><span class="screen-reader-text"><?php _e( 'Close media attachment panel' ); ?></button>
 		</div>
 		<div class="find-box-inside">
 			<div class="find-box-search">