I've just completed porting Hampa Hug's excellent PCE emulator to run in the browser, using Emscripten. I've mainly focused on the pce-macplus build. This is pretty awesome because it means you can run classic Mac OS in the browser. Check it out: PCE.js - Classic Mac OS in the browser.
I've also got the pce-ibmpc build working (emulating an IBM compatible, up to a 286 CPU), and and pce-atarist, an Atari ST emulator also, with browser demos coming soon for each.
A dump of the source is available here but I'll try to clean it up and make it available on GitHub when I get the chance.
Update: source on GitHub, and I've added an IBM PC Demo.