The only part about this written in NaCl is the "ssh" command. The terminal emulator portion of it is entirely JS.
The only reason it isn't currently cross-browser is because I don't have enough time in the day to make it work everywhere. I tried not to make it too Webkit specific though. If some brave JS hacker/Firefox user wants to make this cross browser, I'd be happy to help.
Ah, pretty awesome then. (I can't change my previous reply though, due to time limits)
Now this isn't the same because it's NaCl and therefore not pure-web, but a pure-web terminal emulator would be awesome! It would mean you could embed it within normal web content and mash it up with other stuff. Want to edit your dropbox contents/Gmail in Vim? Maybe pipe the contents of a file into a text box? Maybe just live refresh the output of your web site when you save?
Actually, I'm running out of cool things you could do. Nobody needs a spinning 3D cube with a PDF on it, but the fact that you CAN do it means that people will invent something useful for it, and I have no doubt they would invent something awesome with an in-browser terminal emulator.
Unfortunately, this being NaCl, it may not be it. Shame.