Summary: add support for the CSS UI property 'appearance:none | auto' with
'-webkit-appearance' as an alias. Unship '-moz-appearance'.
'appearance:none' works exactly as '-moz-appearance:none' -- it turns off
the native theme for elements that have one. 'appearance:auto' (the
initial value) makes an element have its default appearance. We are
currently shipping '-moz-appearance' with a large number of other values,
such as 'button', 'range', 'radio' etc. '-moz-appearance' will continue
to work exactly as is, but will now be restricted to UA and chrome style
sheets, i.e. it will *not* be available to web content.
Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1333482
Spec: https://drafts.csswg.org/css-ui-4/#appearance-switching
Platfrom coverage: All
Estimated release: 54 (tentatively)
Preferences: layout.css.appearance.enabled for
'appearance'/'-webkit-appearance' (enabled by default),
layout.css.moz-appearance.enabled for '-moz-appearance' (disabled by
default). All these properties are available to UA and chrome style
sheets though, regardless of the preference settings.
Devtools bug: None needed, I think.
Status in other implementations: No other UA implements the unprefixed
'appearance' as far as I know. Edge implements '-webkit-appearance:none'
but no other values, nor do they implement it unprefixed. WebKit/Blink
implements '-webkit-appearance' with a plethora of values, much like we
currently do for '-moz-appearance'. I don't know what their plans are for
'appearance' and/or restricting the number of supported values.
I think the fact that Edge currently only ships '-webkit-appearance:none'
proves that's all that is needed for web compat. I tend to think we
should also implement the unprefixed property though, because that's what
the CSS spec says and I don't think it'll have any negative impact in
terms of web compat (I admit I'm not 100% certain of that though, but we
can adjust as needed).
Tests: Reftests and mochitests included.
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