Quick update on this: after chatting w/ the MS rep today it sounds like Windows 10 Universal Apps ARE in fact entirely universal, meaning they'll run exactly the same on XB1 as Windows 10. So, as soon as C2 has Windows 10 support, it also has XB1 support - it's the exact same binary - and it sounds like the XB1 will be using the Spartan browser renderer for HTML5/JS-based stuff, including WebGL that's at least as good as with more recent Firefox releases.
Next step: I've got the name & email of the guy headlining the WebGL development, so hopefully he can provide more specific details as to what XB1 will support for UApps.