CSS Color Adjust: Remove 'only' and support used 'dark' for preferred 'light' for color-scheme

Category: CSS
Type: New or changed feature
Status: Enabled by default (Chrome 85)
Intent stage: Evaluate readiness to ship

Summary

The color-scheme property from CSS Color Adjustment level 1 spec changed in two ways. 1. The 'only' keyword is no longer special and is treated as a <custom-ident> as any other unknown color-scheme. The 'only' keyword was previously only allowed in combination with 'light', but had otherwise no effect in Chrome. 2. 'color-scheme: dark' will have a used value of 'dark' even when preferred color-scheme is 'light'. 'color-scheme: light dark' still has a used values based on the preferred scheme.

Motivation

The spec has changed which allows page author to use dark themed UA rendering even when the preferred color-scheme is 'light'. The new behavior is: color-scheme: light -> always light color-scheme: dark -> always dark color-scheme: light dark -> select the preferred scheme This means content which always have a dark theme in their CSS will be able to match that with dark themed UA controls. It also improves interoperability with WebKit which already had this behavior for 'color-scheme: dark'.

Standards & signals

Specification: https://drafts.csswg.org/css-color-adjust-1/#color-scheme-prop
Firefox: No signal — Mozilla were originally opposed to support a used 'dark' value when the preferred scheme was 'light' because user agents using system form control rendering would not be able to use dark themed controls when the system preference was a light, which led to the spec originally being written as it was. The plan for Firefox is to move away from system controls: https://github.com/w3c/csswg-drafts/issues/3881#issuecomment-634463620 From the CSSWG meeting notes for this resolution: "emilio: uneasy about overwriting user preference. Not really objecting"
Safari: Shipped/Shipping — 'only' still has a separate meaning in Safari for opting out of UA color transformations.
Web developers: No signals
Tracking bug: https://crbug.com/1087115

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