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 try to submit the form without filling out a required element. Is there a selector for this? How about for the little message box which pops up?
Yeah as SLaks said there is no CSS selector to do this. I would doubt this will ever be in the scope of CSS because CSS would need to check the contents of an input.
Your best option, still, is probably to call a javascript validation function when clicking a button, rather than actually submitting the form. Then checking the [required] fields for appropriate content and either submitting the form or highlighting the required fields that were not filled in.
JQuery has some nice plugins that take care of this for you http://docs.jquery.com/Plugins/validation