I am actually working on my own game IDE/platform at Play My Code
I'd advise against using WebGL, unless you can fall back onto the canvas (i.e. your building a 2D game). IE 9 doesn't have WebGL support, but it's canvas implementation is very fast and easily the best. There are alternatives you can use to get WebGL support, including a Java based one
which will work in IE 6. However it's very common for these fallback technologies to be incomplete or buggy (like Flash canvas implementations, or the WebGL canvas), so I'd advise you don't use them. That means if you want 3D, either keep it simple (canvas is more then fast enough for a cheap voxel-render or raycaster engine
), or use Java.
You also get cross-browser bugs with both HTML and Java; I find HTML tends to have a lot more, but they are typically minor and easy to fix. With Java it is possible to make an applet which runs at 60fps on Windows 7, crawls at 1fps on XP and Linux, and crashes the entire browser on Mac OS. That said the Canvas is far from stable; I have had to write lots of browser specific code and workarounds in FireFox, Chrome and Safari to solve canvas bugs (IE 9 is the only browser where I have never found an issue). Your game will also be limited to the speed of the slowest browser it will run on (FireFox).
On the flip side Java is _very_ fast, is a much more mature language, has the best tools available I have ever used, scales much better (proper module system through packages, proper multiple file support, and classes are nicer to work with then prototypes), works in IE 6 to 8, and you can use 3D in all browsers.