I use alt="required" for form validation on all my pages with forms and haven't had any trouble with it in any browser. This was a script I downloaded years ago, so it wasn't my invention.
When I add attributes with Dreamweaver, "alt" isn't available with all elements, but it is with input types.
Of course I get a bunch of warnings (that I know I could ignore
The "alt" attribute does not apply for text type inputs (this control defaults to a text control because the type is not specified with the "type" attribute). The "alt", "height", and "src" attributes applies only to image buttons. Consider removing this attribute.
I know you've dropped warnings for similar items that worked and didn't cause problems. This might be another one of them.



