I'll have to add a shameless plug to the visual HTML5 editor in the browser that I've built. Also created with GWT, and using GreenSock JS for animations.
As long as we're doing shameless plugs: checkout my cross-platform product at http://tweenui.com/animator – it's for creating simple HTML5 animations (focus is mobile display ads) in the browser. You can download the result or publish to the cloud.
