1797431 - Kerning is applied to the CJK with "font-kerning: auto".

Status: RESOLVED FIXED

(Core :: Layout: Text and Fonts, defect)

Description

[Takashi Kita]

Attachment: スクリーンショット 2022-10-26 14.23.54.png

Steps to reproduce:

1. open https://tama-san.com/otf_test/
2. check kerning is applied to the CJK part in 「デフォルト」(applied "font-kerning: normal") section.

Actual results:

Kerning('kern') is applied to CJK parts in 「デフォルト」 section.

Expected results:

Kerning('kern') should not be applied to CJK parts in 「デフォルト」 section.

According to OpenType Spec, in CJK Font, the setting value of 'kern' feature is set assuming that it is used together with 'palt'.

https://learn.microsoft.com/en-us/typography/opentype/spec/features_pt#tag-palt
"If 'kern' is activated, 'palt' must also be activated if it exists. "

Comment 1

[BugBot [:suhaib / :marco/ :calixte]]

The Bugbug bot thinks this bug should belong to the 'Core::Layout: Text and Fonts' component, and is moving the bug to that component. Please correct in case you think the bot is wrong.

Comment 2

[Jonathan Kew [:jfkthame]]

(In reply to Takashi Kita from comment #0)

Expected results:

Kerning('kern') should not be applied to CJK parts in 「デフォルト」 section.

I think there are two questions to be considered here.

(1) When font-kerning: normal is used, should the browser automatically apply palt in addition to kern, or is it the author's responsibility to enable that feature? Adobe's description in the feature registry implies that perhaps it should be done automatically (though I think an author should still be able to explicitly override this using the font-feature-settings property).

(2) For CJK content, should we disable kerning by default (when font-kerning: auto is in effect)? The feature registry suggests that kern "should be active by default for horizontal text setting", but your expectation here seems to be that it should not be used by default for CJK text.

Currently, all the browsers I tried leave kerning enabled by default, so the 「デフォルト」 section does have kerning -- and therefore arguably should also have palt applied. Would that be a reasonable default behavior?

Comment 3

[Jonathan Kew [:jfkthame]]

(In reply to Takashi Kita from comment #0)

Steps to reproduce:

1. open https://tama-san.com/otf_test/
2. check kerning is applied to the CJK part in 「デフォルト」(applied "font-kerning: normal") section.

One small correction to the description here, to avoid possible confusion: in the default (デフォルト) section, the value of font-kerning is not normal but auto. This means that kerning "is applied at the discretion of the user agent" (so we could, for example, make it dependent on the script involved), whereas normal requires that kerning is applied.

Comment 4

[Jonathan Kew [:jfkthame]]

Attachment: Result of the example using a patched browser

Here's the result I get after modifying Firefox to (a) disable kerning for CJK scripts when font-kerning is auto, and (b) apply palt by default when kerning is enabled for CJK scripts.

Does this match what you expect/believe is the proper behavior?

Comment 5

[Takashi Kita]

(In reply to Jonathan Kew [:jfkthame] from comment #3)

(In reply to Takashi Kita from comment #0)

Steps to reproduce:

1. open https://tama-san.com/otf_test/
2. check kerning is applied to the CJK part in 「デフォルト」(applied "font-kerning: normal") section.

One small correction to the description here, to avoid possible confusion: in the default (デフォルト) section, the value of font-kerning is not normal but auto. This means that kerning "is applied at the discretion of the user agent" (so we could, for example, make it dependent on the script involved), whereas normal requires that kerning is applied.

(1) When we are using "kerning" in CJK, we expect "palt" is applied. If "font-kering: normal" is in the state that kerning is ON, "font-kerning: normal" should apply "palt" and "kern" at the same time. (Note that for vertical writing, "kern" is changed to "vkrn" and "palt" to "vpal".)
(2) Yes, According to "Requirements for Japanese Text Layout / Principles of Arrangement of Kanji and Kana Characters" (https://www.w3.org/TR/jlreq/#principles_of_arrangement_of_kanji_and_kana_characters), we shouldn't use "kern" or "palt" in default.

(In reply to Jonathan Kew [:jfkthame] from comment #4)

Created attachment 9300333 [details]
Result of the example using a patched browser

Here's the result I get after modifying Firefox to (a) disable kerning for CJK scripts when font-kerning is auto, and (b) apply palt by default when kerning is enabled for CJK scripts.

Does this match what you expect/believe is the proper behavior?

Exactly. Good patch.

(In reply to Jonathan Kew [:jfkthame] from comment #3)

(In reply to Takashi Kita from comment #0)

Steps to reproduce:

1. open https://tama-san.com/otf_test/
2. check kerning is applied to the CJK part in 「デフォルト」(applied "font-kerning: normal") section.

One small correction to the description here, to avoid possible confusion: in the default (デフォルト) section, the value of font-kerning is not normal but auto. This means that kerning "is applied at the discretion of the user agent" (so we could, for example, make it dependent on the script involved), whereas normal requires that kerning is applied.

As you said, default section is "font-kerning: auto", it is default state of typography, so I expect "palt" or "kern" is not applied in CJK text. I was confusing "auto" and "normal", so this topic title is also not correct. I expect "font-kerning: normal" will apply "palt" and "kern", "font-kerning: auto" will apply no features.

Comment 6

[Jonathan Kew [:jfkthame]]

I've opened https://github.com/w3c/csswg-drafts/issues/7958 to suggest that the description of font-kerning in the CSS Fonts spec should be updated to include the points raised here: (a) proportional alternates should be enabled together with kerning; and (b) kerning should not be applied to CJK text by default (when font-kerning: auto is in effect).

(Corresponding Blink bug: https://bugs.chromium.org/p/chromium/issues/detail?id=1378462.)

Comment 7

[Jonathan Kew [:jfkthame]]

Attachment: Bug 1797431 - patch 1 - For CJK text, when kerning is enabled, also apply the OpenType 'palt' feature. r=#layout-reviewers.

As specified in the OpenType feature registry, e.g. at https://learn.microsoft.com/en-gb/typography/opentype/spec/features_ko#tag-kern.

Comment 8

[Jonathan Kew [:jfkthame]]

Attachment: Bug 1797431 - patch 2 - Add a WPT test for the interaction of kerning and proportional-alternates features. r=#layout-reviewers

Depends on D160495

Comment 9

[Jonathan Kew [:jfkthame]]

Attachment: Bug 1797431 - patch 3 - For CJK text, disable kerning by default (when font-kerning:auto is in effect); only apply it when explicitly requested. r=#layout-reviewers

Depends on D160496

Comment 10

[Jonathan Kew [:jfkthame]]

Attachment: Bug 1797431 - patch 4 - Adjust cjk-kerning test to explicitly require kerning to be disabled by default for CJK text. r=#layout-reviewers

Depends on D160497

Comment 11

[Takashi Kita]

Thanks a lot, I confirmed.
Let me know if you need more information.

Comment 12

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/0368c9d06143
patch 1 - For CJK text, when kerning is enabled, also apply the OpenType 'palt' feature. r=layout-reviewers,jwatt.
https://hg.mozilla.org/integration/autoland/rev/a129ec32ac74
patch 2 - Add a WPT test for the interaction of kerning and proportional-alternates features. r=jwatt
https://hg.mozilla.org/integration/autoland/rev/b674f823de1a
patch 3 - For CJK text, disable kerning by default (when font-kerning:auto is in effect); only apply it when explicitly requested. r=jwatt
https://hg.mozilla.org/integration/autoland/rev/842963188422
patch 4 - Adjust cjk-kerning test to explicitly require kerning to be disabled by default for CJK text. r=jwatt

Comment 13

[Web Platform Test Sync Bot (Matrix: #interop:mozilla.org)]

Created web-platform-tests PR https://github.com/web-platform-tests/wpt/pull/36731 for changes under testing/web-platform/tests

Comment 14

[Natalia Csoregi [:nataliaCs]]

https://hg.mozilla.org/mozilla-central/rev/0368c9d06143
https://hg.mozilla.org/mozilla-central/rev/a129ec32ac74
https://hg.mozilla.org/mozilla-central/rev/b674f823de1a
https://hg.mozilla.org/mozilla-central/rev/842963188422

Comment 15

[Web Platform Test Sync Bot (Matrix: #interop:mozilla.org)]

Upstream PR merged by moz-wptsync-bot

Comment 16

[rinsuki]

Hi, I think we shouldn't enable "palt" automatically when "kern" is enabled. since some websites are using font-feature-settings: "kern" instead of font-kerning: normal, and this change automatically enables "palt" in their websites, but I think they didn't expected about "palt" (since any browser didn't automatically enable "palt" previously) and some users (like me) feels difficult to read "palt" text (in my case, it is little, btw).

websites example: Mastodon since v0.6 https://github.com/mastodon/mastodon/commit/49520d6e627e49a1f9f1b8cfa9b323450307fcc6

I'm not sure why they are using this... but possibly EdgeHTML is a one of reason:

in EdgeHTML era (2015~2020), EdgeHTML didn't support font-kerning https://developer.mozilla.org/en-US/docs/Web/CSS/font-kerning but supports font-feature-settings https://developer.mozilla.org/en-US/docs/Web/CSS/font-feature-settings , so they needs to use font-feature-settings for EdgeHTML (I'm not sure about they actually cares EdgeHTML)

Comment 17

[Takashi Kita]

To Assignee,

If the user wants to apply "palt" and "kern" simultaneously in font-feature-settings, the user should already have explicitly specified font-feature-settings: "palt", "kern";, so the state in which "palt" is also applied when specifying font-feature-settings: "kern" may indeed be contrary to the user's intent.

If only font-feature-settings: "kern" is specified, the user may want to reflect only "kern". (For example, a font sales site might want to independently indicate that OpenType "kern" is implemented.)

If "palt" is set when font-feature-settings: "kern" is specified, it is recommended that this be disabled for font-feature-settings.

Comment 18

[Takashi Kita]

If each feature is already applied independently in font-feature-settings, please change the status to CLOSED.

Comment 19

[Takanori MATSUURA]

(In reply to Takashi Kita from comment #17)

To Assignee,

If the user wants to apply "palt" and "kern" simultaneously in font-feature-settings, the user should already have explicitly specified font-feature-settings: "palt", "kern";, so the state in which "palt" is also applied when specifying font-feature-settings: "kern" may indeed be contrary to the user's intent.

If only font-feature-settings: "kern" is specified, the user may want to reflect only "kern". (For example, a font sales site might want to independently indicate that OpenType "kern" is implemented.)

If "palt" is set when font-feature-settings: "kern" is specified, it is recommended that this be disabled for font-feature-settings.

After this fix is applied, the following workaround changeset by bug 1798297 should be fixed.
https://hg.mozilla.org/mozilla-central/rev/99e2e426103b

Comment 20

[Toshihiro Yamada]

At first, the behavior with patches in this bug suit the specification of Opentype.
So, if we want different behaviors against specification, should we open discussion about it?

Comment 21

[Takanori MATSUURA]

(In reply to Toshihiro Yamada from comment #20)

At first, the behavior with patches in this bug suit the specification of Opentype.
So, if we want different behaviors against specification, should we open discussion about it?

Oh I have misunderstood the issue in bug 1798297.
This bug is fixed by changesets in comment 14 and should be closed.

By fixing this bug, "kern" and "palt" is set for CJK fonts including Yu Gothic UI font if font-feature-settings: auto.
This follows the spec of OpenType.

Bug 1798297 has been reported as the regression of this bug.
However it is NOT the regression because the Yu Gothic UI font is one of the CJK fonts.
If the web page author want to apply only "kern" for Yu Gothic UI font, they should be set font-feature-settings: "palt" off.

Comment 22

[Jonathan Kew [:jfkthame]]

Re-closing, as this implements what the OpenType spec currently says. If there are additional concerns about this behavior, please file a new bug with details and specific examples/testcases for further consideration.