Alon Zakai of the Mozilla mobile team has ported DOOM to run in Firefox and Safari, and you can check it out here https://developer.mozilla.org/en-US/demos/detail/doom-on-the-web/:
On his blog (http://mozakai.blogspot.com) Zakai says that
“Emscripten can probably compile most reasonable C/C++ codebases (albeit with some manual intervention in some cases). It is my hope that Emscripten can help against the tendency to write non-web applications, such as native mobile applications (for iOS, Android, etc.) or using plugins on the web (Flash, NaCl, etc.).“
His aim is to make the web a more attractive platform for developers, by letting them use their languages of choice, such as C, C++ or Python.
Zakai’s release notes for DOOM say that it will work, slowly, on Opera, and extremely slow on Chrome due to V8 issue 947.
He also says
“Loading and saving games do work, but only until you leave the page. TODO: Use IndexedDB for persistent storage.”