WIP. Might be part of PowerTools! someday.

Element.Transition.js

(function(){

// thanks @astolwijk
var key = (function(){
	var list = ['', 'webkit', 'Moz', 'O', 'ms'],
		element = document.html;

	for (var i = 0; i < list.length; i++){
	var prefix = list[i];
		if (element.style[prefix ? prefix + 'TransitionProperty' : 'transitionProperty'] != null)
			return prefix.toLowerCase();
	}
	return '';
})();

var Key = key.capitalize(),
	Transition = 'Transition',
	prefix = key ? '-' + key + '-' : '',
	transformKey = prefix + 'transform',
	transition = key ? key + Transition : 'transition',
	animation = key ? key + 'Animation' : 'animation',
	start = 'Start',
	end = 'End',
	iteration = 'Iteration',
	events = Element.NativeEvents;

if (key == 'moz'){
	animation = 'animation';
	transition = 'transition';
	start = start.toLowerCase();
	end = end.toLowerCase();
	iteration = iteration.toLowerCase();
}

events[transition + start] = 2;
events[transition + end] = 2;
events[animation + start] = 2;
events[animation + end] = 2;
events[animation + iteration] = 2;

Element.defineCustomEvent('animationStart', {

	base: animation + start

}).defineCustomEvent('animationComplete', {

	base: animation + end

}).defineCustomEvent('animationIteration', {

	base: animation + iteration

}).defineCustomEvent('transitionComplete', {

	base: transition + end

}).defineCustomEvent('transitionStart', {

	base: transition + start

}).defineCustomEvent('transformComplete', {

	base: transition + end,

	condition: function(event){
		return (event.event.propertyName == transformKey);
	}

});

var methods = {},
	// TODO: fill this list
	useVendorPrefix = {
		transform: 1
	};

['property', 'timingFunction', 'duration', 'delay'].each(function(property){

	var name = property.capitalize();
	methods['get' + Transition + name] = function(){
		return this.getStyle(Key + Transition + name) || '';
	};

	methods['set' + Transition + name] = function(value){
		if (property == 'property') value = value.split(/\s*,\s*/).map(function(item){
			return (useVendorPrefix[item] ? prefix : '') + item;
		}).join(', ');

		this.style[Key + Transition + name] = value || '';
		return this;
	};

});

Element.implement(methods).implement({

	getTransitionProperties: function(){
		return this.getTransitionProperty().split(/\s*,\s*/);
	},

	setTransition: function(properties){
		if (!properties) properties = {};
		if (properties.property != null) this.setTransitionProperty(properties.property);
		if (properties.timingFunction != null) this.setTransitionTimingFunction(properties.timingFunction)
		if (properties.duration != null) this.setTransitionDuration(properties.duration);
		if (properties.delay != null) this.setTransitionDelay(properties.delay);

		return this;
	},

	getTransition: function(){
		return {
			property: this.getTransitionProperty(),
			timingFunction: this.getTransitionTimingFunction(),
			duration: this.getTransitionDuration(),
			delay: this.getTransitionDelay()
		};
	},

	resetTransition: function(){
		return this.setTransition({
			property: '',
			timingFunction: '',
			duration: '',
			delay: ''
		});
	},

	hasTransition: function(){
		return !!(this.getTransitionDuration() || this.getTransitionDelay());
	},

	cancelTransition: function(){
		var properties = this.getTransitionProperties(),
			property, length, i;
		if (properties.contains('all')) properties = Array.slice(this.style); // copy because changing styles reorders

		for (i = 0, length = properties.length; i < length; i++){
			property = properties[i];
			if (property.camelCase() == Key + Transition + 'Duration') continue;
			this.setStyle(property, this.getComputedStyle(property));
		}

		return this;
	},

	// TODO allow object with translate/skew/scale etc.
	transform: function(value){
		return this.setStyle(transformKey, value);
	}

});

}).call(this);

Would be awesome to see this as part of Fx.Morph/Fx.Tween...