As of October 2013, sound only works in Firefox and not in Chrome/Chromium. Emulation speed in Firefox is better than in Chrome, presumably because Firefox has better support for Emscripten's asm.js-based JS code. Also, keymapping has not been done for JSMESS systems, although in principle you should be able to create keymaps in MESS and use them with JSMESS. The joystick is mapped to Alt and the arrow keys by default.
JSMESS development is heading toward its 1.0 version. The [https://github.com/jsmess/jsmess/wiki/JSMESS-v1.0-Systems list of systems] that are supposed to be officially supported by 1.0 also includes the CPC464/664/6128 (although the printer is erroneously listed as the program medium for the CPC on the list).
==CPC demo page==