[css-fonts-5] Computed value of `font-size-adjust: from-font`

The current spec 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 it confirms that this

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

As I'm looking at implementing this in Gecko, the question arose as to whether the behavior in the spec is really what is desired here.

Note that the behavior currently implemented in WebKit (and expected by a WPT testcase) is different. Using current Safari Technology Preview:

> document.body.style.fontSizeAdjust = "from-font"
< "from-font"
> window.getComputedStyle(document.body).fontSizeAdjust
< "from-font"

we see that the computed value is the from-font keyword; it is not resolved to a <number> at computed-value time.

@litherum @svgeesus Should the spec be changed to reflect this, or should we file a bug against WebKit and proceed with implementing what the spec says?

Start with the CSS Fonts 5 sections for the font-size-adjust property and the from-font keyword, then review Gecko bug 1708240, the referenced WebKit behavior, and the relevant WPT testcase. Done means the working group has resolved whether from-font computes to a keyword or a number and the specification and test expectations reflect that decision.