-1.0 would be 100% left, 1.0 would be 100% right, and default would be 0.0 (normal).
(Or better yet, work with WhatWG to get support from other browsers too!)
Backwards compatibility would be fine, as other browsers would continue playing audio from both speakers equally.
I have opened bug at WhatWG to discuss this: http://www.w3.org/Bugs/Public/show_bug.cgi?id=14386
(In reply to comment #1)
> I have opened bug at WhatWG to discuss this: http://www.w3.org/Bugs/Public/show_bug.cgi?id=14386
looks like web audio addresses this request. So this bug can me closed(wont fix) from html5 media side.