Index: src/wp-admin/js/customize-controls.js
===================================================================
--- src/wp-admin/js/customize-controls.js	(revision 31410)
+++ src/wp-admin/js/customize-controls.js	(working copy)
@@ -1535,7 +1535,7 @@
 				response = response.slice( 0, index ) + response.slice( index + signature.length );
 
 				// Create the iframe and inject the html content.
-				self.iframe = $('<iframe />').appendTo( self.container );
+				self.iframe = $('<iframe />', { 'title': wp.customize.l10n.previewIframeTitle } ).appendTo( self.container );
 
 				// Bind load event after the iframe has been added to the page;
 				// otherwise it will fire when injected into the DOM.
@@ -1579,7 +1579,7 @@
 				if ( '1' !== response )
 					reject();
 
-				iframe = $('<iframe src="' + self.previewUrl() + '" />').hide();
+				iframe = $('<iframe src="' + self.previewUrl() + '" title="' + wp.customize.l10n.previewIframeTitle + '" />').hide();
 				iframe.appendTo( self.container );
 				iframe.load( function() {
 					self.triedLogin = true;
@@ -1833,7 +1833,7 @@
 				url:     api.settings.url.login
 			});
 
-			iframe = $('<iframe src="' + api.settings.url.login + '" />').appendTo( this.container );
+			iframe = $('<iframe src="' + api.settings.url.login + '" title="' + api.l10n.loginIframeTitle + '" />').appendTo( this.container );
 
 			messenger.targetWindow( iframe[0].contentWindow );
 
Index: src/wp-includes/js/customize-loader.js
===================================================================
--- src/wp-includes/js/customize-loader.js	(revision 31410)
+++ src/wp-includes/js/customize-loader.js	(working copy)
@@ -114,7 +114,7 @@
 			// Dirty state of Customizer in iframe
 			this.saved = new api.Value( true );
 
-			this.iframe = $( '<iframe />', { src: src }).appendTo( this.element );
+			this.iframe = $( '<iframe />', { src: src, 'title': Loader.settings.l10n.mainIframeTitle }).appendTo( this.element );
 			this.iframe.one( 'load', this.loaded );
 
 			// Create a postMessage connection with the iframe.
Index: src/wp-includes/script-loader.php
===================================================================
--- src/wp-includes/script-loader.php	(revision 31410)
+++ src/wp-includes/script-loader.php	(working copy)
@@ -386,13 +386,15 @@
 	$scripts->add( 'customize-views',    "/wp-includes/js/customize-views.js",  array( 'jquery', 'underscore', 'imgareaselect', 'customize-models' ), false, 1 );
 	$scripts->add( 'customize-controls', "/wp-admin/js/customize-controls$suffix.js", array( 'customize-base' ), false, 1 );
 	did_action( 'init' ) && $scripts->localize( 'customize-controls', '_wpCustomizeControlsL10n', array(
-		'activate'  => __( 'Save &amp; Activate' ),
-		'save'      => __( 'Save &amp; Publish' ),
-		'saveAlert' => __( 'The changes you made will be lost if you navigate away from this page.' ),
-		'saved'     => __( 'Saved' ),
-		'cancel'    => __( 'Cancel' ),
-		'close'     => __( 'Close' ),
-		'cheatin'   => __( 'Cheatin&#8217; uh?' ),
+		'activate'           => __( 'Save &amp; Activate' ),
+		'save'               => __( 'Save &amp; Publish' ),
+		'saveAlert'          => __( 'The changes you made will be lost if you navigate away from this page.' ),
+		'saved'              => __( 'Saved' ),
+		'cancel'             => __( 'Cancel' ),
+		'close'              => __( 'Close' ),
+		'cheatin'            => __( 'Cheatin&#8217; uh?' ),
+		'previewIframeTitle' => __( 'Site preview' ),
+		'loginIframeTitle'   => __( 'Session expired' ),
 
 		// Used for overriding the file types allowed in plupload.
 		'allowedFiles' => __( 'Allowed Files' ),
Index: src/wp-includes/theme.php
===================================================================
--- src/wp-includes/theme.php	(revision 31410)
+++ src/wp-includes/theme.php	(working copy)
@@ -1960,7 +1960,8 @@
 		'isCrossDomain' => $cross_domain,
 		'browser'       => $browser,
 		'l10n'          => array(
-			'saveAlert' => __( 'The changes you made will be lost if you navigate away from this page.' ),
+			'saveAlert'       => __( 'The changes you made will be lost if you navigate away from this page.' ),
+			'mainIframeTitle' => __( 'Customizer' ),
 		),
 	);