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Re: [csswg-drafts] [css-fonts] Reduce layout shift via @font-face descriptor(s) overriding inline spacing (#5533)

From: Xiaocheng Hu via GitHub <sysbot+gh@w3.org>
Date: Mon, 19 Oct 2020 20:40:27 +0000
To: public-css-archive@w3.org
Message-ID: <issue_comment.created-712429292-1603140026-sysbot+gh@w3.org>
We've done some manual testing of two versions of adjustment:
- `advance-override`: Add a fixed amount to all glyph advances
- `advance-proportional-override`: Multiply the glyph advances by a given percentage

(The names are bad but I don't have a better idea...)

[Test result details](https://github.com/w3c/csswg-drafts/files/5404524/cls-with-hacks.html.pdf)

We found one case with legibility issues: when the font is too bold, reducing the advances may squeeze the characters together, making text hard to read. `advance-override` even made all the word boundaries disappear, while `advance-proportional-override` still preserves word boundaries.

Other cases seem fine. Anyway, it would be great if someone professional in Persian or Hindi could take a look at those test cases, as I am very unfamiliar with these scripts.


As a side note, I'm not sure how much we should be concerned about legibility issues. Our main focus is reducing CLS, so in most cases the overridden text should just appear as a flash. It's an issue only when the web font fails to load, and the fallback font differs from the web font so much that the overridden text becomes hard to read, in which case maybe it's better to use a different fallback font.

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