[css-fonts-4] [varfont] "font" shorthand must be updated to incorporate new variation font values #515
Description
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.