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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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9
components/fonts/platform/windows/font.rs
View file

@ -21,7 +21,7 @@ use style::computed_values::font_weight::T as StyleFontWeight;
use style::values::computed::font::FontStyle as StyleFontStyle;
use truetype::tables::WindowsMetrics;
use truetype::value::Read;
use webrender_api::FontInstanceFlags;
use webrender_api::{FontInstanceFlags, FontVariation};
use super::font_list::LocalFontIdentifier;
use crate::{
@ -116,6 +116,7 @@ impl PlatformFontMethods for PlatformFont {
_font_identifier: FontIdentifier,
data: &FontData,
pt_size: Option<Au>,
_variations: &[FontVariation],
) -> Result<Self, &'static str> {
let font_face = FontFile::new_from_buffer(Arc::new(data.clone()))
.ok_or("Could not create FontFile")?
@ -130,6 +131,7 @@ impl PlatformFontMethods for PlatformFont {
fn new_from_local_font_identifier(
font_identifier: LocalFontIdentifier,
pt_size: Option<Au>,
_variations: &[FontVariation],
) -> Result<PlatformFont, &'static str> {
let font_face = FontCollection::system()
.font_from_descriptor(&font_identifier.font_descriptor)
@ -320,4 +322,9 @@ impl PlatformFontMethods for PlatformFont {
Size2D::new(width, height),
)
}
fn variations(&self) -> &[FontVariation] {
// FIXME: implement this for windows
&[]
}
}