Consider changing specified behavior of <meta name=color-scheme> to match WebKit #7213

emilio · 2021-10-13T14:11:26Z

https://html.spec.whatwg.org/#meta-color-scheme says:

If parsed is a valid CSS 'color-scheme' property value, then treat that value as a declaration of the CSS 'color-scheme' property on element's root, cascaded as a presentational hint, and return.

This does match what Chrome (Dev Edition at least) does, but doesn't match what Safari does, see test-case:

<!doctype html>
<meta name="color-scheme" content="dark">
<div style="color-scheme: initial"></div>
<pre><script>
  document.writeln(getComputedStyle(document.documentElement).colorScheme);
  document.writeln(getComputedStyle(document.querySelector("div")).colorScheme);
</script></pre>

I think the Safari behavior (which AFAICT is basically "changing the used value of color-scheme: normal") is slightly preferable, for two reasons:

It allows the initial keyword (which computes to normal) to behave as an author would expect (reverting to the meta-specified color-scheme). With the current definition, once an element sets color-scheme explicitly, the <meta> value is forever lost in that subtree.
It allows <meta name=color-scheme> to affect stuff that doesn't inherit from the root element (like ::backdrop).
It is less weird (I don't think we have precedent for a pres hint on an element based on a meta tag).

The CSS spec is down, but the normal definition says right now:

Indicates that the element doesn’t support color schemes at all, and so the element should be rendered with the browser’s default color scheme. (This is typically, tho not always, similar to light.)

I propose to remove the parenthetical, and perhaps change "browser's default color scheme" by "page's default color scheme". Then HTML would set the "page's default color-scheme" based on the meta tag's value.

This matches, I believe, what WebKit is doing, and seems to be a simpler model for how it should work, IMO. What do you think?

cc @lilles @hober @smfr @dholbert

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tabatkins · 2021-10-13T15:31:38Z

This seems fine to me, if impls are okay with it. It would mean moving the concept of the "browser default" color scheme here into HTML, to cover the case when meta color-scheme isn't specified.

pxlcoder · 2021-10-13T16:52:19Z

Hi @emilio,

I work on WebKit's color-scheme implementation, and can confirm that the behavior you described is what WebKit is doing.

I agree that WebKit's current behavior is preferable to the CSS spec, for all the reasons you mentioned.

lilles · 2021-10-14T10:36:03Z

IIUC, the effect of this change (apart from computed values via CSSOM) is that the used value for the color-scheme would be different for:

Setting color-scheme to normal or initial on an element
::backdrop which is currently not affected by a color-scheme meta since it doesn't inherit the root element

I don't think that should be a back compat concern for Blink.

Color-scheme override probably needs some spec clarifications. In the case where we have:

<meta name="color-scheme" content="dark light">

All elements have color-scheme:normal as their computed values. And the spec says:

"If the element does not support that color scheme, the user agent must also auto-adjust other colors into this chosen color scheme"

If the computed value is normal, the element does not currently count as supporting the dark color scheme, but since normal is causing a used value of dark we should not auto-adjust any colors in that case. Otherwise, that would be a breaking change for Blink.

emilio · 2021-10-14T14:10:44Z

Right, I agree we shouldn't auto-adjust in that case. Seems everyone is ok with such a change? That's awesome :)

I can try to write PRs + tests then :)

Fixes whatwg#7213

Part of whatwg/html#7213 Closes w3c#6726

This allows HTML to "set" the used value of the color-scheme property, via a meta tag. Part of whatwg/html#7213 Closes w3c#6726

Depends on w3c/csswg-drafts#6731 Fixes whatwg#7213.

emilio · 2021-10-15T13:58:39Z

I wrote some PRs:

CSS spec change: [css-color-adjust] Define color-scheme: normal to be hookable by HTML. w3c/csswg-drafts#6731
HTML change: Tweak <meta name=color-scheme> definition. #7226
WPT change: Change wpt tests for <meta name=color-scheme> to not depend on the color-scheme computed value (before whatwg/html#7226) web-platform-tests/wpt#31268 / Assert that color-scheme meta tag doesn't create a pres hint. web-platform-tests/wpt#31271

If everyone is on board (specially @lilles / @tabatkins since Blink would be the one changing behavior) I can file a crbug (and hopefully get these merged).

This allows HTML to "set" the used value of the color-scheme property, via a meta tag. Part of whatwg/html#7213 Closes w3c#6726

Depends on w3c/csswg-drafts#6731 Fixes whatwg#7213.

This allows HTML to "set" the used value of the color-scheme property, via a meta tag. Part of whatwg/html#7213 Closes #6726

Closes #7213. See also w3c/csswg-drafts#6731.

Closes whatwg#7213. See also w3c/csswg-drafts#6731.

The HTML specification changed to use the meta color-scheme as a page's supported color schemes instead of presentation style for the root element. This means the color-scheme meta no longer affects the computed for an initial value root element, but still affects the used color-scheme the same way as before. There is one used value change, and that is when resetting the color-scheme to "normal" down the tree, which now follows the meta element instead of resolving to a "light" color-scheme. We now align with Gecko and Safari. See the whatwg issue[1] for a discussion. [1] whatwg/html#7213 Bug: 1260617 Change-Id: Ic72a2dead75257650f8d99cc5f30f13b7005ea41

The HTML specification changed to use the meta color-scheme as a page's supported color schemes instead of presentation style for the root element. This means the color-scheme meta no longer affects the computed for an initial value root element, but still affects the used color-scheme the same way as before. There is one used value change, and that is when resetting the color-scheme to "normal" down the tree, which now follows the meta element instead of resolving to a "light" color-scheme. We now align with Gecko and Safari. See the whatwg issue[1] for a discussion. [1] whatwg/html#7213 Bug: 1260617 Change-Id: Ic72a2dead75257650f8d99cc5f30f13b7005ea41 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3560051 Reviewed-by: Anders Hartvoll Ruud <[email protected]> Commit-Queue: Rune Lillesveen <[email protected]> Cr-Commit-Position: refs/heads/main@{#987452}

The HTML specification changed to use the meta color-scheme as a page's supported color schemes instead of presentation style for the root element. This means the color-scheme meta no longer affects the computed for an initial value root element, but still affects the used color-scheme the same way as before. There is one used value change, and that is when resetting the color-scheme to "normal" down the tree, which now follows the meta element instead of resolving to a "light" color-scheme. We now align with Gecko and Safari. See the whatwg issue[1] for a discussion. [1] whatwg/html#7213 Bug: 1260617 Change-Id: Ic72a2dead75257650f8d99cc5f30f13b7005ea41 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3560051 Reviewed-by: Anders Hartvoll Ruud <[email protected]> Commit-Queue: Rune Lillesveen <[email protected]> Cr-Commit-Position: refs/heads/main@{#987452}

… schemes, a=testonly Automatic update from web-platform-tests Make color-scheme meta affect all normal schemes The HTML specification changed to use the meta color-scheme as a page's supported color schemes instead of presentation style for the root element. This means the color-scheme meta no longer affects the computed for an initial value root element, but still affects the used color-scheme the same way as before. There is one used value change, and that is when resetting the color-scheme to "normal" down the tree, which now follows the meta element instead of resolving to a "light" color-scheme. We now align with Gecko and Safari. See the whatwg issue[1] for a discussion. [1] whatwg/html#7213 Bug: 1260617 Change-Id: Ic72a2dead75257650f8d99cc5f30f13b7005ea41 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3560051 Reviewed-by: Anders Hartvoll Ruud <[email protected]> Commit-Queue: Rune Lillesveen <[email protected]> Cr-Commit-Position: refs/heads/main@{#987452} -- wpt-commits: 2ffd24a48587d98f03f7f33faa8875ec888035e0 wpt-pr: 33416

Closes whatwg#7213. See also w3c/csswg-drafts#6731.

The HTML specification changed to use the meta color-scheme as a page's supported color schemes instead of presentation style for the root element. This means the color-scheme meta no longer affects the computed for an initial value root element, but still affects the used color-scheme the same way as before. There is one used value change, and that is when resetting the color-scheme to "normal" down the tree, which now follows the meta element instead of resolving to a "light" color-scheme. We now align with Gecko and Safari. See the whatwg issue[1] for a discussion. [1] whatwg/html#7213 Bug: 1260617 Change-Id: Ic72a2dead75257650f8d99cc5f30f13b7005ea41 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3560051 Reviewed-by: Anders Hartvoll Ruud <[email protected]> Commit-Queue: Rune Lillesveen <[email protected]> Cr-Commit-Position: refs/heads/main@{#987452} NOKEYCHECK=True GitOrigin-RevId: 0d60d42899d1eee9b1118700755bc85ae9376476

emilio mentioned this issue Oct 13, 2021

[css-color-adjust] Consider tweaking the specified behavior of color-scheme: normal to support overriding the color-scheme per-page. w3c/csswg-drafts#6726

Closed

annevk added interop Implementations are not interoperable with each other normative change topic: rendering labels Oct 13, 2021

emilio added a commit to emilio/html that referenced this issue Oct 15, 2021

Tweak <meta name=color-scheme> definition.

4e62ade

Fixes whatwg#7213

emilio added a commit to emilio/csswg-drafts that referenced this issue Oct 15, 2021

[css-color-adjust] Define color-scheme: normal to be hookable by HTML.

29f6130

Part of whatwg/html#7213 Closes w3c#6726

emilio added a commit to emilio/csswg-drafts that referenced this issue Oct 15, 2021

[css-color-adjust] Define color-scheme: normal to be hookable by HTML.

80a9661

This allows HTML to "set" the used value of the color-scheme property, via a meta tag. Part of whatwg/html#7213 Closes w3c#6726

emilio mentioned this issue Oct 15, 2021

[css-color-adjust] Define color-scheme: normal to be hookable by HTML. w3c/csswg-drafts#6731

Merged

emilio added a commit to emilio/html that referenced this issue Oct 15, 2021

Tweak <meta name=color-scheme> definition.

7e135db

Depends on w3c/csswg-drafts#6731 Fixes whatwg#7213.

emilio mentioned this issue Oct 15, 2021

Tweak <meta name=color-scheme> definition. #7226

Merged

3 tasks

emilio added a commit to emilio/csswg-drafts that referenced this issue Oct 15, 2021

[css-color-adjust] Define color-scheme: normal to be hookable by HTML.

aee8efe

This allows HTML to "set" the used value of the color-scheme property, via a meta tag. Part of whatwg/html#7213 Closes w3c#6726

emilio added a commit to emilio/html that referenced this issue Oct 15, 2021

Tweak <meta name=color-scheme> definition.

87768fa

Depends on w3c/csswg-drafts#6731 Fixes whatwg#7213.

emilio added a commit to emilio/html that referenced this issue Oct 16, 2021

Tweak <meta name=color-scheme> definition.

62465e6

Depends on w3c/csswg-drafts#6731 Fixes whatwg#7213.

emilio added a commit to w3c/csswg-drafts that referenced this issue Oct 16, 2021

[css-color-adjust] Define color-scheme: normal to be hookable by HTML.

80ca4f9

This allows HTML to "set" the used value of the color-scheme property, via a meta tag. Part of whatwg/html#7213 Closes #6726

domenic closed this as completed in #7226 Oct 18, 2021

domenic pushed a commit that referenced this issue Oct 18, 2021

Tweak <meta name=color-scheme> definition

8e301e8

Closes #7213. See also w3c/csswg-drafts#6731.

dandclark pushed a commit to dandclark/html that referenced this issue Dec 4, 2021

Tweak <meta name=color-scheme> definition

5e6788f

Closes whatwg#7213. See also w3c/csswg-drafts#6731.

chromium-wpt-export-bot mentioned this issue Mar 30, 2022

Make color-scheme meta affect all normal schemes web-platform-tests/wpt#33416

Merged

mfreed7 pushed a commit to mfreed7/html that referenced this issue Jun 3, 2022

Tweak <meta name=color-scheme> definition

2f956fb

Closes whatwg#7213. See also w3c/csswg-drafts#6731.

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Consider changing specified behavior of <meta name=color-scheme> to match WebKit #7213

Consider changing specified behavior of <meta name=color-scheme> to match WebKit #7213

emilio commented Oct 13, 2021 •

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tabatkins commented Oct 13, 2021

pxlcoder commented Oct 13, 2021

lilles commented Oct 14, 2021

emilio commented Oct 14, 2021

emilio commented Oct 15, 2021 •

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Consider changing specified behavior of <meta name=color-scheme> to match WebKit #7213

Consider changing specified behavior of <meta name=color-scheme> to match WebKit #7213

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pxlcoder commented Oct 13, 2021

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