I've finally deployed an update to the HTML5 version that has been in progress for a long time. I'm sure that it still needs some additional testing and fixing, but it is feature complete. Any game can be tested in the HTML5 version by either clicking the "html5" button on your My Games page, or adding "?v=html" to the end of the URL.
The HTML5 engine itself has a lot of fixes since the last update, but the biggest change is that the editor now works in HTML5 as well. Now that the new version is deployed, I expect to be deploying updates much more regularly again.
I'm sure there are things that I've missed, and that the new version will have some issues to iron out, so please post any problems you encounter in this thread and I'll try to sort them out ASAP.