[Webkit-unassigned] [Bug 180073] All text should enable tracking, not just system-ui

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=180073

--- Comment #7 from Myles C. Maxfield <mmaxfield at apple.com> ---
(In reply to zhouqi from comment #6)
> (In reply to Myles C. Maxfield from comment #4)
> > We enable advanced text tracking with system-ui, but don't enable it for
> > other fonts (like PingFang). This is because of performance. The color of
> > all the text in the screen shot you provided is pure black (#000000) - I
> > verified this by opening the screenshot in Photoshop. The weight of the
> > Chinese characters is the same between the two lines, and the weight of the
> > English text is different between the two lines because different fonts are
> > being used to render the English text on the two lines.
> 
> Sorry, the difference of color and font-weight my sensory judgment. 
> 
> So how about the word spacing? This is the biggest problem.
> 
> In addition, if the system-ui is for english only, what is your suggestion?
> In environments other than English, we should use other fonts for English
> display? I think this will cause the English display different in native app
> between web page.

When WebKit sees "system-ui", WebKit substitutes it for a list of constituent fonts, one for each language. One of these constituent fonts is San Francisco, and one is Ping Fang. All of these constituent fonts get advanced tracking turned on.

So, when you say "font-family: system-ui, PingFang SC", WebKit interprets this as "font-family: San Francisco with advanced tracking, (some other fonts with advanced tracking), PingFang with advanced tracking, (more fonts with advanced tracking), PingFang SC". You can see how "system-ui" gets expanded to a list of system fonts with advanced tracking, but anything else in the original font-family list remains untouched.

Similarly, when you say "font-family: PingFang SC, system-ui", WebKit interprets this as "font-family: PingFang SC, San Francisco with advanced tracking, (some other fonts with advanced tracking), PingFang with advanced tracking, (more fonts with advanced tracking)".

This is how multiple languages work with system-ui.

In the first example, Chinese text would be rendered with PingFang with advanced tracking, and English text would be rendered with San Francisco with advanced tracking. However, in the second example, Chinese text would be rendered with PingFang SC (without advanced tracking) and English text would also be rendered with PingFang SC (because PingFang SC supports English and appears earlier in the fallback list). This is causing the difference in tracking and visual weight.

The solution is probably to enable advanced tracking for all fonts, not just the constituent fonts in system-ui. It wouldn't change the visual weight you described, but it would make the tracking the same between the two examples.

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