What I’ve tried successfully –
Use Modernizr to detect Modernizr.input.required, with fallback on custom JavaScript validation if HTML5 validation available. Also had to check for Safari/Opera browser which had unexpected result and had to fall back on custom JavaScript validation as well.
Lastly, I had to also insert a submit button and click on it by JavaScript if I wanted to HTML5 validate, since I want to use an anchor for the form submit button.
With all the trouble of implementing this, as well as inconsistency in validation messages between html5 validate and normal JavaScript validate, is it worth it?
As all of the major browsers try to implement as standard as possible HTML5, the old browsers will not support it ever, so if your app targets old browsers too it is worth the trouble of extending old browser with a self made library which does the same things as HTML5 and the same function names. If this is not your goal, then it is worth it again because it is a good practise ( with HTML 5 people will forget what javascript torture really is ).