[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 #8 from Myles C. Maxfield <mmaxfield at apple.com> ---
(In reply to Myles C. Maxfield from comment #7)
> (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.

I've retitled the issue to reflect this.

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