[Webkit-unassigned] [Bug 259551] New: The `from-font` value for CSS `font-size-adjust` is not computed correctly

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=259551

            Bug ID: 259551
           Summary: The `from-font` value for CSS `font-size-adjust` is
                    not computed correctly
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: jfkthame at gmail.com

Support for the `from-font` value was implemented in https://bugs.webkit.org/show_bug.cgi?id=254790, but does not behave according to the spec.

The current spec[1] says that the computed value of `font-size-adjust` is

    the keyword none, or a pair of a metric keyword and a <number>

and for the `from-font` keyword[2] it confirms that this

    Computes to the <number> corresponding to the specified metric of the first available font.

However, in Safari Tech Preview[3] I'm seeing `from-font` retained as the computed value. This results in incorrect behavior, because when inherited by a child element, it may then resolve to different numbers depending on the font of the child (instead of inheriting the number computed from the metrics of the font on the element where it was specified).

In addition, it appears that the value is not being resolved using the metrics of the first available font *of the element where `font-size-adjust` was specified*, but rather using metrics from the parent element. This can be seen in the demo at https://codepen.io/jfkthame/pen/NWEMaaG, where the use of `from-font` gives different results depending on the font family of the containing block, instead of being computed from the paragraph's own first available font.

See also https://github.com/w3c/csswg-drafts/issues/9050.

[1] https://drafts.csswg.org/css-fonts-5/#font-size-adjust-prop
[2] https://drafts.csswg.org/css-fonts-5/#valdef-font-size-adjust-from-font
[3] Release 171 (Safari 16.4, WebKit 18616.1.15)

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