HTML5 - Validation messages

Question

What element should I use to show validation messages (errors) to the user? Which one is more semantic in HTML5?

Answer 1

I would use javascript to show a javascript error box containing the erro(s).

EDIT: For server side validation we use a <div> that gets filled after an ajax call.

Answer 2

I typically would use a div with an error or notice class to show errors.