Summary: Unprefix the -moz-user-select property, so that it works
without the -moz- prefix.
We happen to be supporting the -webkit- prefixed version of the
property, but other browsers support it also unprefixed, which causes a
lot of confusion.
As part of this work I'm also unshipping the following values from
content (or entirely, if they have no internal usage):
* -moz-all: Was meant to behave as an alias of `all`, but in practice
that's not true. Plus all external usage I found was followed by a
-webkit-user-select: all which would override it.
* -moz-text: It's an internal value which was introduced in bug
1181130. It's only used from contenteditable.css and I haven't
investigated removing it completely, but I'm restricting it to
user-agent stylesheets. I found no relevant external usage.
* tri-state, element, elements, toggle: We parse these but do nothing
with them (lol, I know, right?). They're no longer in the spec so should
be removed.
We also have a non-standard '-moz-none' alias to 'none' which I haven't
investigated removing yet, but probably should in a followup to this bug.
Bug: 1492958 for the removal of non-standard values, 1492739 for the
unprefixing.
Link to standard: https://drafts.csswg.org/css-ui-4/#propdef-user-select
Platform coverage: All
Estimated or target release: FF65
Preference behind which this will be implemented: None
Is this feature enabled by default in sandboxed iframes? Yes
DevTools bug: N/A
Do other browser engines implement this?
This is an interesting question. The current status is:
* Blink supports user-select unprefixed and -webkit- prefixed, with
the same values we'd support after this bug (except our non-standard
-moz-none alias).
* Edge supports the -ms- prefixed version of the property, and the
-webkit- prefixed version. Edge is the only engine to support the
'contain' value.
* WebKit only supports the -webkit- prefixed version.
So all browsers support as of today the -webkit- prefixed version of the
property, which is a fun state of affairs, with a slightly different set
of values.
I think we should try to unprefix sooner than later so this doesn't end
up being something similar to '-webkit-apperance'. Given Chrome supports
the same thing as us unprefixed, I think it's reasonable to do this.
web-platform-tests: Test coverage for all the values is pre-existing.
There's unfortunately little coverage in WPT, but a lot in our selection
and contenteditable tests.
Is this feature restricted to secure contexts? No, as this is merely
unprefixing an existing property.
Thoughts?
-- Emilio
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