[css-sizing-3][css-sizing-4] Move fit-content keyword to Level 3 and fit-content() function to Level 4

[SebastianZ]

Back in 2017, it was resolved to defer the fit-content keyword level 4 of the Sizing spec. together with the stretch keyword.

Though it looks like in the meantime all major browser engines implemented this keyword already. Interestingly, they don't support the stretch keyword yet while the spec. refers to fit-content being equivalent to fit-content(stretch) (which is actually not allowed at the moment, because fit-content() currently only takes a <length-percentage>).

At the same time, the fit-content() function is not supported by any browser so far. There's only an experimental implementation in Firefox.
So, fit-content doesn't seem to be dependent on fit-content().

Therefore, I suggest to revert the resolution of #1912 and move the fit-content keyword back to level 3, and instead defer the fit-content() function to level 4.

Sebastian

[yisibl]

I would suggest migrating the stretch keyword back to Level 3 as well.

• Chrome and Safari have long supported -webkit-fill-available with the prefix -webkit-fill-available.
• Firefox supports -moz-available.
• Recently Chrome has been removing prefixes and is available for testing in Canary.

cc @davidsgrogan