HtmlFormControllsCollection.js

if (typeof HtmlFormControlsCollection == 'undefined') {
	function HtmlFormControlsCollection( arr ) {
		for ( var i = 0; i < arr.length; i += 1 ) {
			this[i] = arr[i];
		}

		// length is readonly
		Object.defineProperty( this, 'length', {
			get: function () {
				return arr.length;
			}
		});

		// a HTMLCollection is immutable
		Object.freeze( this );
	}

	HtmlFormControlsCollection.prototype = {
		item: function ( i ) {
			return this[i] != null ? this[i] : null;
		},
		namedItem: function ( name ) {
			var ret = [];
			for ( var i = 0; i < this.length; i += 1 ) {
				if ( this[i].id === name || this[i].name === name ) {
					ret.push(this[i]);
				}
			}
			return ret.length ? ret : null;
		}
	};
}

var oldElements = HTMLFormElement.prototype.__lookupGetter__('elements');

HTMLFormElement.prototype.__defineGetter__('elements', function() {
	var elems = oldElements.call(this);
	if (typeof HtmlFormControlsCollection !== 'undefined' && elems instanceof HtmlFormControlsCollection) {
		return elems;
	}

	return new HtmlFormControlsCollection(elems);
});