polyfill-service/polyfills/Element/prototype/before/polyfill.js

A polyfill combinator

Document.prototype.before = Element.prototype.before = function before() {
	if (this.parentNode) {
		var args = Array.prototype.slice.call(arguments),
			viablePreviousSibling = this.previousSibling,
			idx = viablePreviousSibling ? args.indexOf(viablePreviousSibling) : -1;

		while (idx !== -1) {
			viablePreviousSibling = viablePreviousSibling.previousSibling;
			if (!viablePreviousSibling) {
				break;
			}
			idx = args.indexOf(viablePreviousSibling);
		}

		this.parentNode.insertBefore(
			_mutation(arguments),
			viablePreviousSibling ? viablePreviousSibling.nextSibling : this.parentNode.firstChild
		);
	}
};

// Not all UAs support the Text constructor.  Polyfill on the Text constructor only where it exists
// TODO: Add a polyfill for the Text constructor, and make it a dependency of this polyfill.
if ("Text" in this) {
	Text.prototype.before = Element.prototype.before;
}