That is if you allow dynamic code loading or such dom manipulation to allow it. But for such cases, you should have already started the fallback engine the first time you scanned through the website code.

But fair enough. In the wild you would have to use fallback pretty much always.

Still, webassembly-able gecko would be handy and would allow for experimenting with above mentioned streamlined 'web standard'. Web author could simply sign it's compliance to the 'standard' using meta tags, http headers or some other way.