New Rule: `no-duplicate-font-family-names`

[przemyslaw-zan]

Rule details

Disallow duplicate names within font families.

What type of rule is this?

Warns about a potential problem

Example code

a {
    /* Duplicate family name */
    font-family: Arial, Arial, sans-serif;

    /* Quoted and unquoted names refer to the same family */
    font-family: "Arial", Arial, sans-serif;

    /* Also checked in the `font` shorthand */
    font: 16px Verdana, Verdana, sans-serif;
}

Prior Art

• font-family-no-duplicate-names

Participation

• I am willing to submit a pull request to implement this rule.

AI acknowledgment

• I did not use AI to generate this issue report.
• (If the above is not checked) I have reviewed the AI-generated content before submitting.

Additional comments

• Comparison would be case-insensitive and quote-insensitive, since family names are matched that way per spec ("Arial" is equivalent to arial).
• font-family: monospace, monospace is a known intentional browser hack (Please explain monospace, monospace one more time :) necolas/normalize.css#519 (comment)), so an allowFontFamilies array option should be included as an escape hatch. Stylelint covers the same need with ignoreFontFamilies.
• The stylelint rule warns that it stumbles on unquoted multi-word font names and names containing escape sequences. This implementation can avoid that limitation: within each comma-separated segment, all consecutive identifiers form a single family name per the grammar (joined with single spaces), and escape sequences can be normalized before comparison, so Times New Roman and "Times New Roman" compare as equal.
• Complements the existing font-family-fallbacks rule. Its helpers (generic font list, CSS-wide keyword checks, font/font-family handling) are currently private to that rule's module - if extracting them to the shared util.js is acceptable, both rules could reuse them.

[DMartens]

I do not think this should be part of the core rules, as I do not think this is a widespread problem.
The duplicate has to follow directly unlike e.g. no-duplicate-import where it can somewhere else in the file making it more likely to happen.
Would like additional opinions from the team.