Note that cpc6128.zip also needs to contain the ROM files from cpc464.zip because unlike regular MESS, JSMESS only loads a single ROM ZIP file per machine.
As of October 2013, sound only works in Firefox and not in Chrome/Chromium. 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.
This [http://mdoege.github.io/jsmess/ JSMESS CPC demo page] (with the SubHunter disc in drive A) also has a ZIP file download of the build.