I knew it was only a matter of time but it looks like some easy-to-use, legit tweening engines are now available for your HTML5 / Javascript needs. Granted, you're going to be on the bleeding edge of animation and may experience some serious frame dropping but it's a start (surprisingly, Canvas does a much better job with animation but isn't as widely adopted by dev's for whatever reason).

• TweenLite JS - probably the most ubiquitous tweening engine for Flash / Actionscript is now available for Javascript. The same familiar syntax should make adoption super easy for anyone, like myself, transitioning away from ever touching Flash again but still interested in diving into some Javascript. HT Madrid.
• Cocos2D HTML5 - you can download the engine here but I'd say visit this demo for an easier to understand version of what's going on. via Will.