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fonts: Implement CSS font-variation-settings property for FreeType platforms (#38642)

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This change adds support for variable fonts via the
[`font-variation-settings`](https://developer.mozilla.org/en-US/docs/Web/CSS/font-variation-settings)
property.

There are three areas where we need to set the variation values:
* Webrender (`compositor.rs`), for drawing the glyphs
* Harfbuzz (`shaper.rs`), for most shaping tasks
* PlatformFont (`fonts/platform/`), for horizontal advances and kerning

For now, freetype is the only platform shaper that supports variable
fonts. I can't easily test the fonts with non-freetype shapers. Thats
why variable fonts are behind the `layout_variable_fonts_enabled` pref,
which is disabled by default.

<img width="1250" height="710" alt="image"
src="https://github.com/user-attachments/assets/1aee1407-f3a2-42f6-a106-af0443fcd588"
/>

<details><summary>HTML test file</summary>

```html
<style>
@font-face {
  font-family: "Amstelvar VF";
  src: url("https://mdn.github.io/shared-assets/fonts/variable-fonts/AmstelvarAlpha-VF.woff2")
    format("woff2-variations");
  font-weight: 300 900;
  font-stretch: 35% 100%;
  font-style: normal;
  font-display: swap;
}

p {
  font:
    1.2em "Amstelvar VF",
    Georgia,
    serif;
  font-size: 4rem;
  margin: 1rem;
  display: inline-block;
}

.p1 {
  font-variation-settings: "wght" 300;
}

.p2 {
  font-variation-settings: "wght" 625;
}

.p3 {
  font-variation-settings: "wght" 900;
}

</style>
<div>
  <p class="p1">Weight</p>
  <span>(font-variation-settings: "wght" 300)</span>
</div>
<div>
  <p class="p2">Weight</p>
  <span>(font-variation-settings: "wght" 625)</span>
</div>
<div>
  <p class="p3">Weight</p>
  <span>(font-variation-settings: "wght" 900)</span>
</div>
</div>
```
</details>



https://github.com/user-attachments/assets/9e21101a-796a-49fe-b82c-8999d8fa9ee1


Testing: Needs decision on whether we want to enable the pref in CI
Works towards https://github.com/servo/servo/issues/37236

Depends on https://github.com/servo/stylo/pull/230

---------

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>
This commit is contained in:
Simon Wülker 2025-08-18 18:30:14 +02:00 • committed by GitHub
parent ce16fbce75
commit 7471ad7730
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GPG key ID: B5690EEEBB952194
19 changed files with 274 additions and 79 deletions
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3
components/fonts/tests/font.rs
View file

@ -27,7 +27,7 @@ fn make_font(path: PathBuf) -> Font {
let data = FontData::from_bytes(&bytes);
let identifier = FontIdentifier::Web(ServoUrl::from_file_path(path).unwrap());
let platform_font = PlatformFont::new_from_data(identifier.clone(), &data, None).unwrap();
let platform_font = PlatformFont::new_from_data(identifier.clone(), &data, None, &[]).unwrap();
let template = FontTemplate {
identifier,
@ -40,6 +40,7 @@ fn make_font(path: PathBuf) -> Font {
style: FontStyle::normal(),
variant: FontVariantCaps::Normal,
pt_size: Au::from_px(24),
variation_settings: vec![],
};
Font::new(FontTemplateRef::new(template), descriptor, Some(data), None).unwrap()
}

12
components/fonts/tests/font_context.rs
View file

@ -133,7 +133,7 @@ mod font_context {
);
},
SystemFontServiceMessage::GetFontInstanceKey(result_sender) |
SystemFontServiceMessage::GetFontInstance(_, _, _, result_sender) => {
SystemFontServiceMessage::GetFontInstance(_, _, _, _, result_sender) => {
let _ = result_sender.send(FontInstanceKey(IdNamespace(0), 0));
},
SystemFontServiceMessage::GetFontKey(result_sender) => {
@ -187,9 +187,12 @@ mod font_context {
path: path.to_str().expect("Could not load test font").into(),
variation_index: 0,
};
let handle =
PlatformFont::new_from_local_font_identifier(local_font_identifier.clone(), None)
.expect("Could not load test font");
let handle = PlatformFont::new_from_local_font_identifier(
local_font_identifier.clone(),
None,
&[],
)
.expect("Could not load test font");
family.add_template(FontTemplate::new(
FontIdentifier::Local(local_font_identifier),
@ -352,6 +355,7 @@ mod font_context {
style: FontStyle::normal(),
variant: FontVariantCaps::Normal,
pt_size: Au(10),
variation_settings: vec![],
};
let family = SingleFontFamily::FamilyName(FamilyName {

2
components/fonts/tests/font_template.rs
View file

@ -36,7 +36,7 @@ fn test_font_template_descriptor() {
.unwrap();
let data = FontData::from_bytes(&bytes);
let handle = PlatformFont::new_from_data(identifier, &data, None).unwrap();
let handle = PlatformFont::new_from_data(identifier, &data, None, &[]).unwrap();
handle.descriptor()
}