Extend XHTML with namespaces

I've seen this somewhere, but I don't remeber the page.

I'd like to extend the HTML tags, so then I can add my own attributes to the HTML elements, I saw a example where the developer added a new xmlns to the page definition, so he could add new things and still开发者_JAVA技巧 be a valid XHTML.

What is the name of this technique and where could I find information about it?

Cheers.

PS: Or maybe I've dreamed this? :D

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Here is an article on this topic. However it would not be "valid" as XHTML is part of a international standard.