whilst WebGL would be fairly comparable to the GL bindings in Glest, it is accessed from Javascript which is running in the browser, and that would mean reimplementing Glest in Javascript.
Google's Chrome browser has 'native code' support and Flash has a C/C++ compiler so you can run C/C++ code in-browser if you massage the code to use the appropriate platform, but its still not the same.
It will be a long time before mainstream gaming is in-browser. I think OnLive approach will happen bigger and sooner for expensive commercial games, and Glest and other full but free games will remain 'native'.
