diff --git a/src/wp-content/themes/twentyfifteen/js/functions.js b/src/wp-content/themes/twentyfifteen/js/functions.js
index ba102a0cb4..f4b80c7e73 100644
--- a/src/wp-content/themes/twentyfifteen/js/functions.js
+++ b/src/wp-content/themes/twentyfifteen/js/functions.js
@@ -14,7 +14,7 @@
 		container.find( '.menu-item-has-children > a' ).after( '<button class="dropdown-toggle" aria-expanded="false">' + screenReaderText.expand + '</button>' );
 
 		// Toggle buttons and submenu items with active children menu items.
-		container.find( '.current-menu-ancestor > button' ).addClass( 'toggle-on' );
+		container.find( '.current-menu-ancestor > button' ).addClass( 'toggle-on' ).attr( 'aria-expanded', 'true' );
 		container.find( '.current-menu-ancestor > .sub-menu' ).addClass( 'toggled-on' );
 
 		container.find( '.dropdown-toggle' ).on( 'click', function( e ) {
@@ -22,12 +22,29 @@
 			e.preventDefault();
 			_this.toggleClass( 'toggle-on' );
 			_this.next( '.children, .sub-menu' ).toggleClass( 'toggled-on' );
-			_this.attr( 'aria-expanded', _this.attr( 'aria-expanded' ) === 'false' ? 'true' : 'false' );
+			_this.attr( 'aria-expanded', _this.hasClass( 'toggle-on' ) ? 'true' : 'false' );
 			_this.html( _this.html() === screenReaderText.expand ? screenReaderText.collapse : screenReaderText.expand );
 		} );
 	}
 	initMainNavigation( $( '.main-navigation' ) );
 
+	// Add unique ID to each .sub-menu and aria-controls to parent links
+	function addUniqueIDToSubMenus() {
+		var subMenus = document.querySelectorAll( '.main-navigation .sub-menu' );
+		subMenus.forEach( function( subMenu, index ) {
+			var parentLi = subMenu.closest( 'li.menu-item-has-children' );
+			subMenu.id = 'sub-menu-' + (index + 1);
+			if ( parentLi ) {
+				var parentLink = parentLi.querySelector( 'button' );
+				if ( parentLink ) {
+					parentLink.setAttribute( 'aria-controls', subMenu.id );
+				}
+			}
+		} );
+	}
+
+	addUniqueIDToSubMenus();
+
 	// Re-initialize the main navigation when it is updated, persisting any existing submenu expanded states.
 	$( document ).on( 'customize-preview-menu-refreshed', function( e, params ) {
 		if ( 'primary' === params.wpNavMenuArgs.theme_location ) {