Comment by Cyberdog

There sort of is. In HTML5, text-based form elements have a new "pattern" attribute which takes a regular expression that matches valid input, so the browser can do client-side validation without using JavaScript to intercept the form before it's posted and such. Assuming the site developers have bothered to implement it on their site, then theoretically a password manager could use that to determine valid characters for generated passwords (or, at least, invalid ones). I don't know if any of them actually do this, though.

http://www.w3.org/wiki/HTML/Elements/input/password