simple focus-within polyfill

focus-within.js

(function(window, document){
	'use strict';
	var slice = [].slice;
	var removeClass = function(elem){
		elem.classList.remove('focus-within');
	};
	var update = (function(){
		var running, last;
		var action = function(){
			var element = document.activeElement;
			running = false;
			if(last !== element){
				last = element;
				slice.call(document.getElementsByClassName('focus-within')).forEach(removeClass);
				while(element && element.classList){
					element.classList.add('focus-within');
					element = element.parentNode;
				}
			}
		};
		return function(){
			if(!running){
				requestAnimationFrame(action);
				running = true;
			}
		};
	})();
	document.addEventListener('focus', update, true);
	document.addEventListener('blur', update, true);
	update();
})(window, document);