[whatwg] Separating intrinsic sizing out from the iframe seamless attribute

[Ojan Vafai]

I made a proposal for this at the end of
http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2012-July/036584.html.
The best way to get this specced is to provide additional use-cases (for
having intrinsic sizing without inheriting style) in addition to use case
#3 in my email.