In bug 1655859, I intend to disable parsing for ::-moz-focus-outer on
This pseudo-element used to be an <input type=range> specific
pseudo-element to show some focus outlines, but it does literally
nothing anymore (other than being parsed), after the changes in bug
1636998. As part of making it do nothing, I made it not parse on
Nightly, two releases ago, and we've had no reported regression. All
uses that I've been able to find involve a single selector, so there
should be no behavior change in that regard.
Let me know if there's any concern with doing this. This is behind a
pref (layout.css.moz-focus-outer.enabled), so we could revert if need be.