There are a dozen of opensource javascript game engines that you could use. I've bookmarked a few javascript engines here http://pinboard.in/u:akash/t:gameengine Akihabara according to what I've heard, is the most stable out there, but the last time I tried, the documentation was lacking (although they do have a few tutorials which a lot of these gameengines don't have).
I'm mentioning javascript engines because you said you needed something cross platform including mobile. This way you could just just wrap the game with Adobe Air for desktop platforms and maybe native wrapper for mobile platforms. You might want to look into tools like Phonegap and Titanium
1) Sound doesn't work on every platform/browser but this is not impact's fault, it's html5.
2) Performance on android isn't good. It works ok in Opera, but not well the default browser.
3) In-browser multitouch on android is not available yet.
And here's another article from IBM about offline web/mobile apps in a more general sense: http://www.ibm.com/developerworks/web/library/wa-offlineweb/...
Hopefully these can help you out in some way!
http://code.google.com/p/forplay/
ForPlay is a cross-platform game abstraction library for writing games that compile to:
Desktop Java
HTML5 Browsers
Android
Flash