I wrote a simple form, including text field, textarea, select and some validating by WP_error. When I try to submit the form without filling text field or textarea, I got a popup tip pointing the blank text field, it says "please fill in the field". This is different from the error message I wrote, and I didn't write any ajax for validating-- my validating messages comes after the form submitted and redirected. Is this validating part of WP3.3 new feature? or it's a firefox feature?
Edit: Thanks to joseph, found the reason-- it's html5 new feature-- required attribute
The required attribute applies to all form controls except controls with the type hidden, image inputs, buttons (submit, move-up, etc), and select and output elements. For disabled or readonly controls, the attribute has no effect.