HTML5 Required input styling

Question

In HTML 5, we can mark inputs as required and then select them with the [required] pseudo-selector in CSS. But I only want to style them when they

Answer 1

There's a :user-error pseudoclass in the CSS Selectors 4 working draft that will do exactly this, firing on both input blur and form submit.

In the mean time, I'm personally using the awesome webshims polyfill library which covers :user-error, or you could hack it out yourself with something along the lines of Toby's answer.