Are custom elements valid HTML5?

Question

I\'ve been unable to find a definitive answer to whether custom tags are valid in HTML5, like this:

Hello!

Answer 1

To give an updated answer reflecting modern pages.

Custom tags are valid if either,

1) They contain a dash

2) They are embedded XML

Hello!

This assumes you are using the HTML5 doctype

Considering these simple restrictions it now makes sense to do your best to keep your HTML markup valid (please stop closing tags like and

---

, it's silly and incorrect unless you use an XHTML doctype, which you probably have no need for).

Given that HTML5 clearly defines the parsing rules a compliant browser will be able to handle any tag that you throw at it even if it isn't strictly valid.