[Webkit-unassigned] [Bug 248656] New: `system-ui` should not be treated as a font face name in `src:local(...)`

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=248656

            Bug ID: 248656
           Summary: `system-ui` should not be treated as a font face name
                    in `src:local(...)`
           Product: WebKit
           Version: Safari 15
          Hardware: Unspecified
                OS: macOS 12
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Text
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: jfkthame at gmail.com
                CC: mmaxfield at apple.com

Steps to reproduce the problem:

1. Load testcase at https://codepen.io/jfkthame/pen/dyKgwPv
2. Observe rendering of line "Test 1".

The example uses a @font-face rule with `src: local(system-ui)`. However, "system-ui" is not the name of an available font face on the system; it's a generic CSS font *family* (not *face*) name. Therefore, the `local(system-ui)` source should be ignored and the next listed source used instead.

Safari displays the "Test 1" line using the system UI font family (".SFNS").

Some quotes from the relevant spec text at https://drafts.csswg.org/css-fonts-3/#font-face-name-value:

> The locally-installed <font-face-name> argument to local() is a format-specific string that uniquely identifies a single font face within a larger family.
> For OpenType and TrueType fonts, this string is used to match only the Postscript name or the full font name in the name table of locally available fonts.
The CSS generic family names clearly don't fit this at all, and therefore they should not "work" in `src:local(...)`.

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