I think that HTML5 is the way of the future for the development of Ria application due to the reduction of the ammount of javascript need for basic animation, video, etc. So what are the tools available NOW to develop html5 interfaces?
EDIT: It's been a year since this question has been asked. Has the outlook for HTML5 changed for the better? Or is Bobince right, and is HTML5 still premature as a viable RIA platform?
Forget “HTML5” as a single monolithic entity. The spec is insanely long and complicated (it is unlikely any browser will manage to implement everything) and it's a long way from being finalised. Some of the stupider features have already been removed, but it remains to be seen what will stick and meet wide implementation.
So it's pretty meaningless at the moment to ‘author to HTML5’, an indeterminate soup of new features.
However the HTML5 effort is eventually spitting out some smaller, more manageable APIs for the stuff that has browser manufacturer acceptance. Features like video/audio, canvas, non-cookie storage, Web Sockets, better cross-window scripting and simple but long-needed methods like getElementsByClassName and querySelector. Some of these have been forked into their own documents at W3, and some of them you can use today (though preferably with backup code for IE and other old browsers).