Even if IE9/10 perfectly conformed to every standard, you still have to account for 7 and 8, as well as firefox 2 and 3 none of which have modern standards implemented either. Which shouldn't matter anyway since you should have graceful fallbacks for older browsers, regardless, and always use the more compatible approach.

A lot of these problems also come from the fact that designers have a tendency to abuse the new tags in HTML5 such as the section tag... Which is doubly hilarious since when I see other people use it, they're using it as a replacement for a div apply styles for the header/content/footer sections which is funny because the W3C standards specifically state that if you're using it for styling reasons you should use a div instead (and there's no benefits to using section instead, anyway).

I don't even understand why this is so hard for people. We're talking about a data mark up format... Not a programming language. It's not that hard to maintain compatibility (even for IE6 if need be)