I suspect that a platform for rendering streaming applications (which is basically what this would be, if "web apps" are its primary purpose) would probably not use CSS/HTML at all. The purpose of those is to display and format a subset of XML documents.
I would imagine all of the rendering would be done natively, and browsers would be replaced by VMs, and the DOM with some sort of cross-platform application library like SDL.
So basically, like Java or Flash, but language agnostic (rather than serving HTML, sites would serve source code that compiles to WebAssembly, or just the WASM itself) and maybe with something like a dependency manager for common libraries or runtimes.
