This was briefly discussed before: http://meta.math.stackexchange.com/q/10500/ Yes, the HTML tag is
<blockquote>, so using it for visual separation of question from surrounding material is somewhat improper. But this usage quite common, in fact it predates Math.SE itself: the model question on MathOverflow (written at the early days of the site) uses
> to isolate the actual question.
By the way, SE itself promotes the use of
<blockquote> for something other than quotes: namely, spoilers are realized as
Personally, I think that mimicking a $\LaTeX$ environment in MarkDown is easy enough.
Question. Is every finitely generated foo isomorphic to bar?
After all, we don't usually see highlighted blocks of text in mathematical writing, apart from some low-level undergraduate textbooks.
It may be more semantic to use HTML headers, but the choice SE gives us is between
Way too big
Still too big
Smaller than regular bold