Currently a number of HTML5 form features are being landed but they apparently do not impact form submission. E.g. the required attribute is supported but a user can simply submit a form that is using it. This will very likely lead to interoperability issues and may even make that particular part of the feature impossible to implement later on, which would be bad.
(Was not quite sure who to cc, hope you don't mind mjs.)
This is actually quite a big deal — I hadn't realised for example that required="" as implemented in WebKit didn't block submission. If this is accurate, then we run the risk of people using it and relying on the way it doesn't block submission... and then we can't add the ability to block submission in future.
A patch to prevent submission is under review; Bug#34930
I don't think the current status of WebKit is so harmful. DOM API such as checkValidity() is useful without preventing submission.
The problem is that sites may start to rely on the browser not performing validation so you can no longer implement it. However, if you think this will be fixed before that happens there is no problem as far as I can tell.
The patch of Bug#34930 was committed. This issue doesn't exist anymore.