What properties login form needs to have to trigger "remember password" for all (or most) browsers?

Background: For years I have been using Opera, and the rule has been that almost anything that looks like login form triggers this feature. Now I'm exploring other browsers, and surprisingly find that one of login forms I visit most often, Roundcube login page, does not trigger this feature neither in Chromium nor Firefox (tried various versions and page setups of RC).

Since it's on my VPS, I could patch it up and eventually even send that to RC development team, but I have the unnerving feeling that this must have been solved over and over, and it's just me being blind or something.

So again: is there any consensus between browser developer as to when this featue should be triggered? Is there a best practice for webmasters to tell the browser "this is the login page"? Or are all browsers doing their own heuristics?