Index: wp-admin/css/customize-controls.css
===================================================================
--- wp-admin/css/customize-controls.css	(revision 41286)
+++ wp-admin/css/customize-controls.css	(working copy)
@@ -228,6 +228,37 @@
 	border-left-color: #0073aa;
 }
 
+/* CSS Tooltips */
+#customize-controls .tooltip {
+	position: relative;
+	overflow: visible;
+}
+
+#customize-controls .tooltip:after {
+	display: none;
+	position: absolute;
+	bottom: 100%;
+	right: 50%;
+	border: 1px solid #ddd;
+	margin-bottom: 5px;
+	padding: 5px 8px;
+	background: rgba(27,31,35,0.8);
+	color: #fff;
+	content: attr(aria-label);
+	text-align: center;
+	transform: translateX(50%);
+	white-space: pre;
+	opacity: 0;
+}
+
+#customize-controls .tooltip:hover:before,
+#customize-controls .tooltip:focus:before,
+#customize-controls .tooltip:hover:after,
+#customize-controls .tooltip:focus:after {
+	display: inline-block;
+	opacity: 1;
+}
+
 #accordion-section-themes + .control-section {
 	border-top: 1px solid #ddd;
 }
Index: wp-admin/customize.php
===================================================================
--- wp-admin/customize.php	(revision 41286)
+++ wp-admin/customize.php	(working copy)
@@ -185,13 +185,12 @@
 							continue;
 						}
 						$active = ! empty( $settings['default'] );
-						$class = 'preview-' . $device;
+						$class = 'tooltip preview-' . $device;
 						if ( $active ) {
 							$class .= ' active';
 						}
 						?>
-						<button type="button" class="<?php echo esc_attr( $class ); ?>" aria-pressed="<?php echo esc_attr( $active ) ?>" data-device="<?php echo esc_attr( $device ); ?>">
-							<span class="screen-reader-text"><?php echo esc_html( $settings['label'] ); ?></span>
+						<button aria-label="<?php echo esc_html( $settings['label'] ); ?>" type="button" class="<?php echo esc_attr( $class ); ?>" aria-pressed="<?php echo esc_attr( $active ) ?>" data-device="<?php echo esc_attr( $device ); ?>">
 						</button>
 					<?php endforeach; ?>
 				</div>