How do you style the HTML5 form validation messages?

Update: Chrome does not allow styling form validation bubbles anymore: https://code.google.com/p/chromium/issues/detail?id=259050

In the latest iterations of Chrome, support has been added for pseudo selectors for these, namely;

::-webkit-validation-bubble{}
::-webkit-validation-bubble-top-outer-arrow{}
::-webkit-validation-bubble-top-inner-arrow{}
::-webkit-validation-bubble-message{}

For more information on these, check out the Webkit "Styling Form Controls" trac page.

Additionally, firefox has support for the element attribute x-moz-errormessage which enables you to change the text of the error message, which is something you could do in Chrome using CSS to and -webkit-validation-bubble-message. See more about x-moz-errormessage on the MDN Docs page.

As of yet, Firefox has no way to style the error bubbles.

You'll need to use more specific selectors for everything else I'm afraid.. body > div etc.