Add CSS Mixins explainer

[John Jansen (thejohnjansen)]

Creating an explainer for the work we're doing for CSS-Mixins-1. The current explainer is a publicly hosted document that contains outdated information and includes zincing (@functions). We should have a new one limited down to Mixins.

Old Explainer: https://css.oddbird.net/sasslike/mixins-functions/

[Copilot]

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Copilot review overview

Review effort: Lite
Findings: 2

Open (2)

• Marking a newly-added explainer as Archived is likely to confuse readers about whether this… · New
• The PR description references CSS-Mixins-1, but this link filters issues by label:css-mixins-2.… · New

What changed in this PR

Adds a new in-repo explainer for the CSS Mixins work and links it from the main README, replacing reliance on an older externally hosted explainer that includes outdated functions-focused content.

Changes:

• Add a “CSS Mixins” entry to the README explainer index
• Introduce a new CSSMixins/explainer.md documenting the current mixins authoring model and status

File	Description
README.md	Adds the CSS Mixins explainer to the repository’s explainer index
CSSMixins/​explainer.md	New explainer document describing the current CSSWG direction for CSS mixins

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[Copilot]

Copilot review overview

🟡 Changes recommended

The explainer contains outdated draft-status text and incorrect internal and ChromeStatus links.

Review effort: Balanced
Findings: 1

Open (1)

• Broken table-of-contents link to references section · New

Resolved since last review (2)

• The PR description references CSS-Mixins-1, but this link filters issues by label:css-mixins-2.…
• Marking a newly-added explainer as Archived is likely to confuse readers about whether this…

Previously missed (2)

In code that hasn't changed since last review

Outdated claim that the draft does not define mixins

CSSMixins/​explainer.md:23

This status is already outdated: the current Editor’s Draft contains normative @mixin, @apply, and @private sections. Saying that it does not define mixins misleads readers about the proposal’s current maturity; retain only the experimental-status caveat.

Incorrect ChromeStatus feature ID in link

CSSMixins/​explainer.md:49

This uses the Chromium tracking issue ID as a ChromeStatus feature ID. The CSS Mixins ChromeStatus entry linked from the TAG review is 5108022310469632; point this feature-entry link there instead.

This issue also appears on line 215 of the same file.