[riskable]

You mean like Gate One (https://github.com/liftoff/GateOne)? Right now it's just an HTML5 terminal/SSH client but soon it will also do X11 (video demo):

http://www.youtube.com/watch?v=6zJ8TNcWTyo

It will basically do the same thing as Mainframe2 except it'll be open source and you'll be able to run it (the server) wherever you like. I've been working on the X11 support quite a lot over the past few weeks... Hopefully the public beta will be ready soon.

[mtdewcmu]

That's cool. I can't foresee too many issues in putting an X server in the browser. Except that if you left the page, all of the clients would get killed in a fairly rough fashion. Could you run a window manager?