Wouldn't that entail dropping support for a number of currently widely used browsers, such as MS IE 9? For instance, using the "required" attribute on a text input field is not supported by MS IE 9.
Unless of course you mean: we support html 5 but fall back to javascript if necessary. In Ruby on Rails they've shown a few neat tricks to do that: using javascript, scan the dom for a required attribute and implement the necessary validation machinery (again, in javascript) based on that piece of information.