Fix false positives for system fonts in font-family-no-missing-generic-family-keyword

[regseb]

Which issue, if any, is this issue related to?

Closes #3793

Is there anything in the PR that needs further explanation?

No, it's self explanatory.

[jeddy3]

@regseb Thanks for your contribution.

Is there somewhere in the specs we can find out more about these font values? I believe the current list is based on the one found in this spec.

[regseb]

@jeddy3 I got list from MDN. Here are the spec.

[jeddy3]

Fascinating! Thanks for the sharing the link to the spec. I had no idea there were such things are system fonts and they are set using the font shorthand property which is sometimes not a shorthand property!

Here's the relevant bit from the spec for those interested:

System fonts may only be set as a whole; that is, the font family, size, weight, style, etc. are all set at the same time. These values may then be altered individually if desired. If no font with the indicated characteristics exists on a given platform, the user agent should either intelligently substitute (e.g., a smaller version of the 'caption' font might be used for the 'small-caption' font), or substitute a user agent default font. As for regular fonts, if, for a system font, any of the individual properties are not part of the operating system's available user preferences, those properties should be set to their initial values.

That is why this property is "almost" a shorthand property: system fonts can only be specified with this property, not with 'font-family' itself, so 'font' allows authors to do more than the sum of its subproperties. However, the individual properties such as 'font-weight' are still given values taken from the system font, which can be independently varied.

[Emphasis mine]

[jeddy3]

This PR LTGM. Nice work :)

[ntwb]

Thanks @regseb 👍

[ntwb]

Chengelog updated in: 384f201

Fixed: false positives for system fonts in font-family-no-missing-generic-family-keyword (#3794).