I first noticed this on https://www.reddit.com/r/rust/comments/6o7cqf/this_week_in_rust_191/ Steps to reproduce: * Enable Stylo * Navigate to data:text/html,<input><input type=submit style=border:none> * Click inside the text field to give it focus * Press the Tab key once Expected result (and result with Stylo disabled): the button gets focus, which is made visible by a dotted border/outline. Actual result in 56.0a1 (2017-07-20): the button gets focus (pressing Enter would submit the form if there were one), but there is no visual indication of that. (At least on Linux, border:none forces the button to be rendered entirely by Gecko and with CSS rather than as a "native" or native-looking widget. The visual indication appears correctly on "native" buttons, even with Stylo.) It looks like the CSS rules responsible for this visual indication are in layout/style/res/forms.css. Simplified: input[type="submit"]::-moz-focus-inner { border: 1px dotted transparent; } input[type="submit"]:-moz-focusring::-moz-focus-inner { border-color: ButtonText; } Messing with obj-x86_64-pc-linux-gnu/dist/bin/chrome/toolkit/res/forms.css in a local build, I found that both :-moz-focusring and ::-moz-focus-inner appear to be working correctly when used independently, but a rule with :-moz-focusring::-moz-focus-inner is never applied.

Yup, this is pretty much https://bugzilla.mozilla.org/show_bug.cgi?id=1382222