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

It's possible and allowed:

User agents must treat elements and attributes that they do not understand as semantically neutral; leaving them in the DOM (for DOM processors), and styling them according to CSS (for CSS processors), but not inferring any meaning from them.

http://www.w3.org/TR/html5/infrastructure.html#extensibility-0

But, if you intend to add interactivity, you'll need to make your document invalid (but still fully functional) to accomodate IE's 7 and 8.

See http://blog.svidgen.com/2012/10/building-custom-xhtml5-tags.html (my blog)