Recently I worked on a website with a lot of forms, and decided to use HTML5 form validation to make my life easier, with a JS fallback for older browsers. It turns out that Webkit added support for HTML5 form validation, then found a problem, so they switched off the implementation without switching off the exposed API. So every JS fallback that tests for the presence of the HTML5 methods on form elements will assume that HTML5 form validation is implemented, and leave the validation up to the browser... which doesn't actually do it at all.
That means server-side validation, which of course one needs to do anyway. But it's not to hard to imagine somebody making a site during the brief window when Chrome supported HTML5 form validation, and then discovering their site broken in Chrome a few months later.
They didn't "switch it off" they merely turned off form submission blocking when the form was invalid. It's back on in chrome now because they have the error notification bubble. The constraint validition API, the error pseudo-classes and the extra attributes all work in Safari.
If the answer to "are the contents of this form valid" is always "yes", I don't really care how much of the validation API the browser supports; it's effectively going to waste.
I'm glad to hear it's turned back on; I hope the form-validation interface is as sleek and professional-looking as the one in Firefox 4.
That means server-side validation, which of course one needs to do anyway. But it's not to hard to imagine somebody making a site during the brief window when Chrome supported HTML5 form validation, and then discovering their site broken in Chrome a few months later.