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Here are all the tests I could perform:

• macOS 26.3 - English
  • Chrome (v145): Tested pages in various languages, no visual change after this PR
  • Firefox (v147): Same as above
  • Safari (v26.3): Same as above
• Windows 10 Pro - English
  • Chrome (v145): Tested pages in various languages, no visual change after this PR
  • Firefox (v147): Same as above
  • Edge (v145): Same as above
• iOS 26.3 - English
  • Safari (v26.3): Tested pages in various languages, no visual change after this PR
• ipadOS 26.3 - English
  • Safari (v26.3): Tested pages in various languages, no visual change after this PR
• Endeavour OS virtual machine hosted on Windows - English (English host)
  • Firefox (v146): Tested pages in various languages, some visual change after this PR

Sharing here screenshots of the visual change observed in Firefox on Endeavour OS virtual machine:

Language	Before	After
en
ja
zh-cn

Sharing here screenshots of the visual change observed in Firefox on Endeavour OS virtual machine:

Very interesting! The Latin font change seems like an improvement aesthetically maybe? Hard to tell tbh. I’d be curious why it changes here, what the before/after fonts are, and whether we could/should handle them differently?

Very interesting! The Latin font change seems like an improvement aesthetically maybe? Hard to tell tbh. I’d be curious why it changes here, what the before/after fonts are, and whether we could/should handle them differently?

Yeah, I am planning more investigations tomorrow morning but had to run for today. I'll share here my findings.

Regarding the actual changes, difficult to say, but I think the "before" is closer to what I'm currently seeing on all other OS?

Hi, I'm Japanese and Windows user. I confirmed that the proper font is applied in Japanese.
If there is other check list that I should check the preview site, I'll check them.!

	before	after
top page
document page

After:

(Japanese + Firefox + sans-serif = Source Han Sans VF ≒ Noto Sans CJK)

I think it's fine as long as it's within the scope of simple adjustments.

FYI: (Windows + Chrome + additional manual adjustments (word-break: auto-phrase + font-feature-settings: "palt") for <h1> here + sans-serif = Noto Sans JP)

We do not need to add these properties to this PR, I think.

Firefox 147.0.4, Windows 11 25H2, zh-CN

Language	Production	Preview
zh-cn
ja
en

I have fonts preferences default on browser except having mono font set to JetBrains Mono.

Microsoft YaHei is the default font for UI and texts in Simplified Chinese Windows systems.
p.s. I have Noto Sans SC installed but since the variable --__sl-font does not include Noto Sans SC but Noto Sans, it is not applied to the site and yet the production site still appears in Microsoft YaHei in any language.

it is not applied to the site

It is because Firefox has ridiculously declined to adopt Noto Sans CJK fonts unlike Chromium and is just a expected behavior. FYI, in Japanese sites, Meiryo (Firefox) or Noto Sans JP (Chromium) instead of Yu Gothic will be used after you install Japanese Supplemental Fonts:

Add-WindowsCapability -Online -Name "Language.Fonts.Jpan~~~und-JPAN~0.0.1.0"

This has already been installed in all Japanese Windows machines.

While Yu Gothic is a high-quality font, it can be challenging to fully leverage its potential.

yet the production site still appears in Microsoft YaHei in any language.

It depends on the browser's or OS's preferred languages. If no CJK language is in the list, a Simplified Chinese font, namely Microsoft YaHei (UI), will be assigned. In my environment, Japanese is at the top, so Yu Gothic UI is assigned instead.

Endeavour OS seems to use the monolithic Noto Sans CJK font as the fallback CJK font in its UI. Linux-based OSes are not affected by this problem as long as they adopt Noto CJK fonts as their UI fonts. However, some cellphone manufacturers seem to adopt other Chinese-or-Japanese-dedicated fonts as UI fonts and trigger this bug.

Do you know what fonts are designated for the sans-serif in Firefox and the default UI font in your Endeavour OS? Have you ever been changed them since install?

Investigated a little bit more.

First, to explain how it works on Linux (at least on EndeavourOS), is that the /etc/fonts/conf.d directory will contains files or symlinks to files that contain XML configuration with different priorities that mostly define font aliases. Such configuration can be tested using the fc-match command, e.g. fc-match "Courier" will return the font that will be used when "Courier" is requested, depending on the configured aliases and font availability.

Current website

The font used for the latin characters is "Noto Sans".

With our previous font stack (before this PR), the first match found is system-ui in a /etc/fonts/conf.d/60-latin.conf file:

<!--
  system-ui
-->
<alias>
  <family>system-ui</family>
  <prefer>
    <family>Adwaita Sasns</family>
    <family>Cantarell</family>
    <family>Noto Sans UI</family>
    <family>Segoe UI</family>
    <family>Segoe UI Historic</family>
    <family>Segoe UI Symbol</family>
  </prefer>
</alias>

On a default EndeavourOS installation, the first installed font that matches this alias is "Noto Sans".

Deploy preview

The font used for the latin characters is "Liberation Sans".

With the new font stack (from this PR), the first match found is Arial in a /etc/fonts/conf.d/30-metric-aliases.conf file:

<!-- Microsoft -->
<alias binding="same">
  <family>Arial</family>
  <accept>
    <family>Arimo</family>
    <family>Liberation Sans</family>
    <family>Albany</family>
    <family>Albany AMT</family>
  </accept>
</alias>

On a default EndeavourOS installation, the first installed font that matches this alias is "Liberation Sans" and as Arial is defined right before 'Noto Sans in the font stack, it is the one that is now used.

Following that logic, I also tested installing Chrome on EndeavourOS and the behavior is the same: the difference is also visible and the same font are used before and after compared to Firefox.

I see. I think Arial could be stripped or less of a priority nowadays. (Windows → Segoe UI / Apple → -apple-system)

As I expected, a Chinese font (YaHei or Source Han Sans) is now used even in Japanese Windows (Firefox):

Chrome:

As I expected, a Chinese font (YaHei or Source Han Sans) is now used even in Japanese Windows (Firefox):

@tats-u Is this in the deploy preview? So it’s a problem with this PR?

This is an expected behavior in the Preview of this PR. It is a Chinese page.

This is an expected behavior in the Preview of this PR. It is a Chinese page.

Oh, great, perfect — thank you! Confused myself after looking at so many screenshots 😄

Thank you to everyone who has been sharing. It is very helpful!

Here are more results following the change from 069ec02:

• Endeavour OS virtual machine hosted on Windows - English (English host)
  • Firefox (v146): Tested pages in various languages, no visual change after this PR
  • Chrome (v145): Same as above
• Fedora 43 virtual machine hosted on Windows - English (English host)
  • Firefox (v147): Tested pages in various languages, no visual change after this PR
  • Chrome (v145): Same as above
• Ubuntu 24.04.4 virtual machine hosted on Windows - English (English host)
  • Firefox (v147): Tested pages in various languages, some visual change after this PR
  • Chrome (v145): Same as above

Sharing here screenshots of the visual change observed in Firefox and Chrome on the Ubuntu 24.04.4 virtual machine:

Language	Before	After
en
zh-cn

• Current website: the font used for the latin characters is "Ubuntu Sans".
• Deploy website: the font used for the latin characters is "Noto Sans".

In this case, this is explained by Ubuntu/Gnome overriding system-ui to include the Gnome default font, which is "Ubuntu Sans". In the deploy preview, system-ui is no longer used so we fall back in the stack until we reach "Noto Sans", like on the other distributions I tested.

In this case, this is explained by Ubuntu/Gnome overriding system-ui to include the Gnome default font, which is "Ubuntu Sans". In the deploy preview, system-ui is no longer used so we fall back in the stack until we reach "Noto Sans", like on the other distributions I tested.

This seems like an acceptable change I guess? In theory, we could add Ubuntu Sans to the font stack, but I did a quick search and this does not seem to be a common thing people do, so probably not necessary.

It would be endless if we were to list every default UI font for each OS individually.

This seems like an acceptable change I guess? In theory, we could add Ubuntu Sans to the font stack, but I did a quick search and this does not seem to be a common thing people do, so probably not necessary.

From the quick research I did, yeah, I came to the same conclusion and I think this may be acceptable.

The font in zh-cn looks great together

@liruifengv What is your OS? Have you tweaked the font config in your browser? Probably it's the behavior I expect.

@liruifengv What is your OS? Have you tweaked the font config in your browser? Probably it's the behavior I expect.

MacOS and Windows. I don't remember if I changed the font.

MacOS:

Windows:

Windows Chrome config:

@liruifengv Thank you. It is the behavior that we exactly expect.

Windows Chrome config:

Probably you have not changed them. Noto is default in Chromium-based browsers if installed. Windows bundles Noto fonts that match with your language. Those of the other languages are required to be manually appended: https://learn.microsoft.com/windows/deployment/windows-missing-fonts?pivots=windows-11 (Noto are not listed there but probably included)