fix(w3c/style): always add the dark stylesheet link so the theme toggle appears

[marcoscaceres]

Closes #5200

W3C's fixup.js only injects the light/dark/auto theme toggle when it finds a dark stylesheet link, since it drives that link's disabled property. Light-only specs never got the link, so the whole block was skipped and no toggle appeared. The link is now always added, disabled when the spec has not opted into dark mode.

Intended consequence worth calling out: a light-only spec now gets a working toggle, so a reader can switch it to dark and the dark stylesheet will apply, on a document that never declared color-scheme. That is deliberate. Readers get the same control they have on every other TR document, and spec authors should not be deciding this on their behalf.

Two implementation notes for review. The link is disabled after insertion rather than before, because per HTML the disabled setter is a no-op while the link's style sheet is still null. And the test asserts only that the link exists, not that it is disabled, since disabled depends on the sheet loading from www.w3.org and would make the test network-dependent; the rel="stylesheet" filter in the selector matters, because a preload hint for the same href is also present.

Verified in Firefox 153, the browser in the bug report: without the fix the sidebar has no dark stylesheet link, no toggle and no radio inputs; with it, the toggle and all three options appear.

Written with AI: this change was generated by Claude. Per AI_POLICY.md.

[Copilot]

Pull request overview

This PR updates the W3C styling pipeline so a dark.css stylesheet link is always present, ensuring W3C’s fixup.js injects the light/dark/auto theme toggle even for “light-only” specs (closing #5200).

Changes:

• Always add the W3C dark.css stylesheet link; keep it disabled when the spec hasn’t opted into dark mode via meta[name="color-scheme"].
• Add a regression test asserting that a rel="stylesheet" dark stylesheet link exists for light-only specs (without asserting disabled to avoid network dependence).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/w3c/style.js	Always injects a dark stylesheet link (disabled for light-only specs) so W3C fixup can render the theme toggle.
tests/spec/w3c/style-spec.js	Adds a test to ensure light-only specs still get a dark stylesheet link[rel="stylesheet"] so the toggle can appear.

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[marcoscaceres]

Confirmed visually. After fix:

[marcoscaceres]

Approving. Reviewed locally fully together with Claude and had copilot check it.