Wide characters in rustdoc: Fall-back to default CJK serif font on Windows is ugly

In Windows, browsers like Chrome and Firefox use legacy fonts as default serif font to render.
rustdoc uses its custom serif fonts, but these do not support CJK glyph, so the browser renders CJK characters as default one.

The problem is that the default serif font in Windows is so ugly and not clear.
The default sans-serif font looks just ok.

Take a look at this screenshot after matching the pixels on the monitor.

Why don't we just use sans-serif font for rustdoc? Or, we can use sans-serif fonts that look like serif font.

With Windows default serif font (batang.ttc):

With Noto Sans font (not serif):

I'm using Windows 10 20H2 (Korean) and Firefox 87.0 with clean installation.

The label would be T-rustdoc, but I cannot add it.

[workingjubilee]

In Windows, browsers like Chrome and Firefox use legacy fonts as default serif font to render.
rustdoc uses its custom serif fonts, but these do not support CJK glyph, so the browser renders CJK characters as default one.

The problem is that the default serif font in Windows is so ugly and not clear.
The default sans-serif font looks just ok.

Take a look at this screenshot after matching the pixels on the monitor.

Why don't we just use sans-serif font for rustdoc? Or, we can use sans-serif fonts that look like serif font.

Huh. I have a recently installed Windows, and on both Firefox and Edge, I see:

This looks much more similar to your second image. I wonder if #83156 or #82315 affected this? Or maybe it is because I do not have batang.ttc installed? Windows says the only Korean font I have is "Malgun Gothic". Having a good CJK default is ideal, so I am very interested in finding out exactly what is going on here.

Legacy fonts like batang.tcc maybe bundled in Korean Windows.

"Malgun Gothic" (malgun.ttf) is default sans-serif font for Korean, so your browser maybe rendered that font instead of batang.tcc.

[workingjubilee]

Oh I see, that makes sense. So because it can't find a Korean serif font, it might have used a Korean sans serif font.
Checking... yep, Firefox says that it is using Malgun Gothic instead on https://docs.rs/xingapi/0.2.6/xingapi/, and obviously "have no meh-looking Korean serif fonts already installed" is not really a reasonable expectation.

BTW, user-defined custom theme can solve this issue for crate-level. Pass --extend-css argument to rustdoc with custom css.

For example,

@import url("https://fonts.googleapis.com/css2?family=Noto+Sans+KR:wght@300&display=swap");

body {
  font-family: "Noto Sans KR", sans-serif;
  line-height: 1.5;
}

pre.rust.rust-example-rendered {
  line-height: 1.4;
}

[jsha]

Would it work to add Noto Sans CJK as a custom font-face, and then include it in the fallback list, like:

body {
	font: 16px/1.4 "Source Serif 4", "Noto Sans CJK", serif;
	...
}

@jsha Noto sans fonts that support CJK exist only for OpenType. Also its size is too big ;) (20MB)

So we should provide webfonts for each language instead of providing a CJK OpenType font.
For example, Noto Sans HK, JP, KR, SC, TC...