the obvious problem is that it's going to take a generation of browsers before everyone has native <canvas> support along with a respectable Javascript engine.
IE6 market share is still 17% with IE7 at ~25% (source: http://www.w3schools.com/browsers/browsers_stats.asp)
I can't imagine advanced Processing/JS is going to run well without native canvas and SpiderMonkey/V8/Squirrelfish