index.js

try {
	// test for scope support
	document.querySelector(':scope *');
} catch (error) {
	(function (ElementPrototype) {
		// scope regex
		var scope = /:scope(?![\w-])/gi;

		// polyfill Element#querySelector
		var querySelectorWithScope = polyfill(ElementPrototype.querySelector);

		ElementPrototype.querySelector = function querySelector(selectors) {
			return querySelectorWithScope.apply(this, arguments);
		};

		// polyfill Element#querySelectorAll
		var querySelectorAllWithScope = polyfill(ElementPrototype.querySelectorAll);

		ElementPrototype.querySelectorAll = function querySelectorAll(selectors) {
			return querySelectorAllWithScope.apply(this, arguments);
		};

		// polyfill Element#matches
		if (ElementPrototype.matches) {
			var matchesWithScope = polyfill(ElementPrototype.matches);

			ElementPrototype.matches = function matches(selectors) {
				return matchesWithScope.apply(this, arguments);
			};
		}

		// polyfill Element#closest
		if (ElementPrototype.closest) {
			var closestWithScope = polyfill(ElementPrototype.closest);

			ElementPrototype.closest = function closest(selectors) {
				return closestWithScope.apply(this, arguments);
			};
		}

		function polyfill(qsa) {
			return function (selectors) {
				// whether the selectors contain :scope
				var hasScope = selectors && scope.test(selectors);

				if (hasScope) {
					// fallback attribute
					var attr = 'q' + Math.floor(Math.random() * 9000000) + 1000000;

					// replace :scope with the fallback attribute
					arguments[0] = selectors.replace(scope, '[' + attr + ']');

					// add the fallback attribute
					this.setAttribute(attr, '');

					// results of the qsa
					var elementOrNodeList = qsa.apply(this, arguments);

					// remove the fallback attribute
					this.removeAttribute(attr);

					// return the results of the qsa
					return elementOrNodeList;
				} else {
					// return the results of the qsa
					return qsa.apply(this, arguments);
				}
			};
		}
	})(Element.prototype);
}