Does HTML5 support namespaces?

Question

Are we allowed to extend HTML5 with new tags like <foo:bar> in HTML5?

I ask because the Facebook API includes just this kind of thing^**, and our pages are defined as HTML5.

More specifically, is it possible to make a HTML5-conformant page that uses fb:fbml?

Answer 1

Strictly speaking, no. You can have JavaScript code write the FBML which gets around any potential validation issues, but it's bad form.

That aside, even if you could, you don't want to make a page that uses FBML. FBML will be deprecated on Jan 1, 2012 and after June 1, 2012 FBML will no longer work.

Source: https://developers.facebook.com/docs/reference/fbml/

Answer 2

Use XHTML(5). XML allows namespaces, and XHTML(5) is an extension of XML, so therefore, it allows namespaces, while HTML(5) does not.