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[css-inline] The web de-facto requires nonnegative line gap metrics in fonts #5064

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litherum opened this issue May 12, 2020 · 5 comments
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[css-inline] The web de-facto requires nonnegative line gap metrics in fonts #5064

litherum opened this issue May 12, 2020 · 5 comments
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Closed Accepted by CSSWG Resolution css-inline-3 Current Work css-inline-4

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@litherum
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Firefox and Chrome both run web fonts through the OpenType Sanitizer, which rewrites the font file. One of the things it does is clamp line gap metrics so that they aren't negative.

WebKit got a compatibility bug about this, since WebKit wasn't doing the same clamping. I've committed a change in WebKit to align with the other browsers and perform the clamping.

Because now all the browsers agree, and it is de-facto required for the web, the CSS fonts spec should say so (somewhere).
@litherum litherum added the css-fonts-4 Current Work label May 12, 2020
@astearns
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Is the line gap metric clamping the only thing this sanitizer does that you think is required for web compat?

Are these metrics allowed to be negative per the OpenType spec?

@litherum
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Is the line gap metric clamping the only thing this sanitizer does that you think is required for web compat?

This is the only thing I'm aware of today. Perhaps we will discover more in the future.

Are these metrics allowed to be negative per the OpenType spec?

The spec says they are of signed type. It doesn't say anything about negative values. https://docs.microsoft.com/en-us/typography/opentype/spec/hhea and https://docs.microsoft.com/en-us/typography/opentype/spec/os2

@css-meeting-bot
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The CSS Working Group just discussed [css-fonts] The web de-facto requires nonnegative line gap metrics in fonts, and agreed to the following:

  • RESOLVED: Fonts with negative line gaps are clamp at 0
The full IRC log of that discussion <dael> Topic: [css-fonts] The web de-facto requires nonnegative line gap metrics in fonts
<dael> github: https://github.com//issues/5064
<dael> myles: Pretty straightforward. Browsers do it but not in spec.
<dael> myles: If font says it has negative line-gap it's clampped at 0
<dael> Rossen_: Anyone with different opinion why this shouldn't be the case?
<dael> Rossen_: Obj to Fonts with negative line gaps are clamp at 0
<dael> florian: Quesiton, have you checked this is true of print formatter?
<dael> myles: No, haven't checked. FF, Chrome, and WK do it
<dael> florian: Sounds like a good idea, wondering if done in print and how we deal with compat. Maybe just suggest they do it as well
<dael> Rossen_: This is why we have standards
<dael> RESOLVED: Fonts with negative line gaps are clamp at 0
<dael> myles: I think metric is within inline spec so I think this issue should be parented under that spec
<dael> Rossen_: Sure

@mozilla-apprentice mozilla-apprentice mentioned this issue Apr 28, 2021
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@litherum litherum changed the title [css-fonts] The web de-facto requires nonnegative line gap metrics in fonts [css-inline] The web de-facto requires nonnegative line gap metrics in fonts Apr 28, 2021
@litherum
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Oh, this probably also affects the grammar of line-gap-override in css-fonts

@litherum
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Oh, this probably also affects the grammar of line-gap-override in css-fonts

Oh, looks like this was already done in 348ed35

@svgeesus svgeesus assigned svgeesus and unassigned svgeesus Jun 9, 2021
@svgeesus svgeesus added css-inline-4 and removed css-fonts-4 Current Work labels Jun 9, 2021
@fantasai fantasai added the css-inline-3 Current Work label Feb 3, 2022
fantasai added a commit that referenced this issue Apr 1, 2023
@fantasai
[css-inline-3] Floor line gap metric at zero. #5064
c8a4b3f
@fantasai fantasai closed this as completed Apr 1, 2023
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Closed Accepted by CSSWG Resolution css-inline-3 Current Work css-inline-4
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