Add -webkit-line-clamp

[annevk]

Per https://bugzilla.mozilla.org/show_bug.cgi?id=866102 Edge has already added it and there seems increasing pressure on Firefox to add it too. 😟

cc @RByers

[domenic]

Per https://bugzilla.mozilla.org/show_bug.cgi?id=866102#c12 this seems like another case, like -webkit-appearance, where there's an ideal version that CSSWG editors would prefer, and there's the version that everyone is being forced to implement for web compat. Not sure what to do there.

https://bugzilla.mozilla.org/show_bug.cgi?id=866102#c23 and https://bugzilla.mozilla.org/show_bug.cgi?id=866102#c28 implies Mozilla might be interested in trying the CSSWG version first at least.

[miketaylr]

From @heycam (in Comment 29)

Once we have an implementation of the standards version we can look at whether we really need to implement the oddities of -webkit-line-clamp.

If and when we implement the standard version, I'd recommend we at least do some testing to see if aliasing -webkit-line-clamp will work for most existing usage. Getting sites to remove prefixes is pretty much a lost cause.

[heycam]

-webkit-line-clamp is now defined in css-overflow-3.

https://drafts.csswg.org/css-overflow-3/#webkit-line-clamp

So I think this issue can be closed.

[miketaylr]

nice!