HTML5 and CSS3 aren’t exactly officially supported yet, but the HTML5 doctype is completely harmless and backward compatible. So, I say might as well start using it since it will become the standard anyway. And I think they prepared the doctype to be permanent, so even with HTML6, 7, 8 etc it will still be:
which is very nice.
I’ve written and designed several website templates and WordPress themes using respected and up-to-date standards: