Hi PI48895,
I think content/language type is more likely to cause issues especially with character encoding like &, ©, etc. Code might break and there will be other validation issues if your content/code doesn’t match the doctype that represents it. I actually recommend the HTML5 doctype and UTF-8 for content.
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:
<!DOCTYPE html>
which is very nice.
I’ve written and designed several website templates and WordPress themes using respected and up-to-date standards:
Website Templates: http://freewebplate.org/
WordPress Themes: http://tidythemes.com/
New WordPress theme I just released: http://calmestghost.com/groundwork/
Thanks, Bryan