[css-fonts-4] [varfont] "font" shorthand must be updated to incorporate new variation font values

[litherum]

Now that the syntax for font-weight, font-stretch, and font-style is extended, the shorthand must be updated to not make the parsing rules ambiguous.

The most straightforward way of doing this is to make the font shorthand eliminate any of the new values accepted by existing properties. However, we may be able to do better than this, and allow web authors at least some variation support in the "font" shorthand.

[litherum]

The shorthand can't attribute percentages to font-stretch because they need to be attributed to font-size instead. Therefore, the shorthand should only accept the font-stretch keywords, not the number values.

This logic is probably applicable to all the variation numeric values.

[litherum]

Syntactically, the shorthand can take raw numbers for font-weight and <angle>s for font-style (but not <percentage>s for font-width).

I see no reason why the shorthand shouldn't be updated to accept this.

[litherum]

Because "0" lengths don't require a unit, distinguishing between a weight of 0 and a size of 0 requires lookahead.

[litherum]

See also: 89583ac