Browsers which support HTML5 form validation are "suppose," to validate the input prior to submitting the forms. I think this trended a bit ago here on HN about email validation...even if you do a <input type="email">, a user can still submit an email in these forms (blah@you , 123@12.zz , 12345@localhost ,...get the idea).

Another quirk about form validation in HTML5 is the forms won't get validated UNTIL the user submits the form and then the browser may highlight the error in consecutive order (1 at a time o_O), imagine your poor UX for a user that has multiple errors.

Except that the form is already validated client side by the input elements, that's the point. It doesn't make any sense to validate it twice client side.

Cross browser compatibility is fine without putting in javascript (which people frequently fail to test cross browser and ends up being wrong and broken). Modern browsers get the best experience, old crap browsers get a degraded but still functional experience. You don't need to try to make everything the same everywhere, just make it work everywhere.