Bug 283388 – all font family members downloaded when glyph missing

• Status: NEW
• Component: Text

[Cameron Beccario]

Created attachment 473280 [details]
document that reproduces the font loading problem using Google Fonts API

Steps to reproduce:
- Using @font-face at-rules, define a font family F that contains multiple files that differ only by font weight. For example, 100, 400, and 700. Do not specify unicode-range.
- Style the document body to have font-weight 400 and font-family F with a generic fallback.
- Add some characters to the document body that do not map to any glyphs in F.

Expected:
The 400 weight file for F is downloaded and then the generic fallback is used.

Actual:
*All* weights for F are downloaded and then the generic fallback is used.

Build Date & Hardware:
- MacBook Pro M1 Max, Sonoma 14.6.1
- Release 207 (Safari 18.2, WebKit 19621.1.4.3)

Both Chrome and FF download only a single font.

It appears that Webkit's font loading algorithm searches for the missing glyph in all members of the font family before trying the next family. This is non-conformant with the CSS3 font matching procedure (https://drafts.csswg.org/css-fonts-3/#font-style-matching):

"‘font-weight’ is matched next, so it will always reduce the matching set to a single font face... If no matching face exists or the matched face does not contain a glyph for the character to be rendered, the next family name is selected."

[Cameron Beccario]

Created attachment 473281 [details]
screenshot showing rendered webpage and associated network downloads

[Cameron Beccario]

Related issues:

https://bugs.webkit.org/show_bug.cgi?id=42154
https://bugs.webkit.org/show_bug.cgi?id=113715
https://bugs.webkit.org/show_bug.cgi?id=168114

[Radar WebKit Bug Importer]

<rdar://problem/140599230>