Seems like it could be appropriate to both. Any thoughts?
The general consensus thus far is that we're going to have a bit of overlap between SO and SF, possibly a little with SU. Just because a question would be equally appropriate on any of the other trilogy sites does not mean it's not welcome here.
As long as the question is in the context of what is on topic here, and not "99% programming 1% SEO", it should be perfectly fine. In other words, finding some way to make a question about SEO just because it did not get answers on SO might be frowned upon by some members of the community. However, if the ultimate (and original) goal of the question has something to do with running a website, it's fine.
After we get out of beta, it is very likely that we will be able to migrate questions to and from SO and SF, but I can't be quoted on that.
Depends on the context of the question. That person is asking about SEO implications of using
display:none; so I would say it totally belongs here.
If they were asking about the technical aspect of the CSS style (for example, how does it affect DOM rendering) then that would be for doctype, so on and so forth.