Index: src/wp-admin/customize.php
===================================================================
--- src/wp-admin/customize.php	(revision 31320)
+++ src/wp-admin/customize.php	(working copy)
@@ -181,7 +181,7 @@
 			</a>
 		</div>
 	</form>
-	<div id="customize-preview" class="wp-full-overlay-main"></div>
+	<div id="customize-preview" class="wp-full-overlay-main" title="<?php esc_attr_e( 'Site preview' ); ?>"></div>
 	<?php
 
 	// Render control templates.