Easier said than done. For a trivial combination, it's easy. For most real world requirements it becomes a combinatorial explosion of various strategies that are mutually exclusive in many parts.

Unless you have some new machine learning approach to work out all the platform quirks and arrive at pure fallback nirvana?

Doesn't GWT have something that does pretty much exactly that ?

Use progressive enhancement, not fallbacks.

http://en.wikipedia.org/wiki/Progressive_enhancement

I am curious, how many people would prefer this fallback overr the actual site. I'd do.