*/
			$amp_sub_menu = $sub_menu->cloneNode( true );

			/** @var Element $amp_expand_button */
			$amp_expand_button = $expand_button->cloneNode( true );

			$sub_menu->setAttribute( Attribute::CLASS_, $sub_menu->getAttribute( Attribute::CLASS_ ) . ' display-on-desktop' );
			$expand_button->setAttribute( Attribute::CLASS_, $expand_button->getAttribute( Attribute::CLASS_ ) . ' display-on-desktop' );

			$menu_item->appendChild( $amp_expand_button );
			$menu_item->appendChild( $amp_sub_menu );
			$this->update_twentynineteen_main_nested_menu( $amp_sub_menu );

			$amp_sub_menu->setAttribute( Attribute::CLASS_, $amp_sub_menu->getAttribute( Attribute::CLASS_ ) . ' expanded-true display-on-mobile' );
			$amp_expand_button->setAttribute( Attribute::CLASS_, $amp_expand_button->getAttribute( Attribute::CLASS_ ) . ' display-on-mobile' );

			// Handle buttons.
			$amp_expand_button->addAmpAction( 'tap', "$sidebar_id.open" );
			$back_button = $this->get_first_element( $xpaths['close_button_in_submenu'], $amp_sub_menu );
			if ( ! empty( $back_button ) ) {
				$back_button->addAmpAction( 'tap', "$sidebar_id.close" );
			}

			$amp_nested_menu->appendChild( $amp_sub_menu );
			$amp_sidebar->appendChild( $amp_nested_menu );
			$menu_item->appendChild( $amp_sidebar );
		}
	}

	/**
	 * Update the markup of the nested menu to AMP compatible markup.
	 *
	 * @param Element $sub_menu Element of sub-menu from main navigation.
	 *
	 * @return void
	 */
	public function update_twentynineteen_main_nested_menu( Element $sub_menu ) {

		$xpaths = [
			'menu_items'    => './li[ contains( @class, "menu-item" ) and contains( @class, "menu-item-has-children" ) ]',
			'expand_button' => './button[ @class = "submenu-expand" ]',
			'back_button'   => './li[ contains( @class, "mobile-parent-nav-menu-item" ) ]//button[ contains( @class, "menu-item-link-return" ) ]',
			'submenu'       => './ul[ @class = "sub-menu" ]',
		];

		$menu_items = $this->dom->xpath->query( $xpaths['menu_items'], $sub_menu );
		if ( 0 === $menu_items->length ) {
			return;
		}

		/** @var Element $menu_item */
		foreach ( $menu_items as $menu_item ) {
			$nested_sub_menu = $this->get_first_element( $xpaths['submenu'], $menu_item );

			if ( empty( $nested_sub_menu ) ) {
				continue;
			}

			$this->update_twentynineteen_main_nested_menu( $nested_sub_menu );

			$back_button = $this->get_first_element( $xpaths['back_button'], $nested_sub_menu );
			if ( ! empty( $back_button ) ) {
				$back_button->setAttribute( Attribute::AMP_NESTED_SUBMENU_CLOSE, '' );
			}

			$open_button = $this->get_first_element( $xpaths['expand_button'], $menu_item );
			if ( ! empty( $open_button ) ) {
				$open_button->setAttribute( Attribute::AMP_NESTED_SUBMENU_OPEN, '' );
			}

			$amp_nested_menu_div = AMP_DOM_Utils::create_node(
				$this->dom,
				Tag::DIV,
				[
					Attribute::AMP_NESTED_SUBMENU => '',
				]
			);

			$nested_sub_menu->setAttribute( Attribute::CLASS_, $nested_sub_menu->getAttribute( Attribute::CLASS_ ) . ' expanded-true' );
			$amp_nested_menu_div->appendChild( $nested_sub_menu );
			$menu_item->appendChild( $amp_nested_menu_div );
		}
	}
}