[css-ui] Make appearance: menulist-button match WebKit/Chromium #3551
Description
See https://bugzilla.mozilla.org/show_bug.cgi?id=1481615
I believe this value would cause a drop-down select to render with its CSS styles (including a down-arrow button), like when you specify a border or a background. Firefox currently makes it equivalent to 'menulist'.
Possibly both approaches are web-compatible enough, but I think we might as well match WebKit/Chromium on this since this "appearance" is needed anyway when border/background is applied.