ENH: tree-shake unused AMP CSS custom properties + trim @font-face to woff2

[hotinglok]

Builds on #14229. Further reduces the size of the inline <style amp-custom> on AMP article pages to keep well under AMP's 75KB CSS limit. This is also applied to lite pages.

Adds two CSS optimisation passes for the AMP/Lite inline <style> and wires them into _document:

• treeshakeCssCustomProperties — drops unused :root custom properties (service themes declare full palette/font-token sets, but a page uses only a fraction).
• trimFontFaceSourcesToWoff2 — Trims @font-face src lists down to their woff2 source only. Service themes ship each face with a woff2 source plus a legacy woff
  fallback, e.g.

src:
  url('…BBCReithSans_W_Bd.woff2') format('woff2'),
  url('…BBCReithSans_W_Bd.woff') format('woff');

Impact and Testing

Against the real news theme (latin, 4 font faces, ~6.1KB theme CSS) with the actual transforms:

Transform	Saving
tree-shake custom properties	~4.7–5.0KB
woff2 trim	~0.4KB

Running the following script in browser console to check the ampCustom size:

const ampCustom = document.querySelector('style[amp-custom]').textContent;
console.log('amp-custom size:', ampCustom.length, 'bytes');

On latest (as of 05/08/2026): 61822 bytes
On this branch: 57252 bytes

Tree-shake is the dominant lever: components consume theme colours via compile-time SCSS $ variables (literal hex), so the theme's --brand-*/palette custom properties have effectively no var() consumers in the rendered CSS and are safely removed.

[Copilot]

Pull request overview

This PR aims to further reduce AMP article inline CSS size (to stay under AMP’s 75KB amp-custom limit) by introducing two new CSS post-processing utilities: (1) tree-shaking unused :root custom properties and (2) trimming @font-face src lists to woff2-only.

Changes:

• Added treeshakeCssCustomProperties utility (+ unit tests + README) to remove unused :root custom properties, including transitive dependency retention.
• Added trimFontFaceSourcesToWoff2 utility (+ unit tests + README) to keep only woff2 (and local()) in @font-face src.
• Updated _document.page.tsx to replace the shared optimiseCssPrefixes import with a new inline regex-based implementation.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
ws-nextjs-app/utilities/trimFontFaceSourcesToWoff2/README.md	Documents the new woff2-trimming utility.
ws-nextjs-app/utilities/trimFontFaceSourcesToWoff2/index.ts	Implements @font-face src trimming logic.
ws-nextjs-app/utilities/trimFontFaceSourcesToWoff2/index.test.ts	Adds unit tests for woff2 trimming behavior.
ws-nextjs-app/utilities/treeshakeCssCustomProperties/README.md	Documents the new custom-property tree-shaker.
ws-nextjs-app/utilities/treeshakeCssCustomProperties/index.ts	Implements :root custom-property tree-shaking with dependency closure.
ws-nextjs-app/utilities/treeshakeCssCustomProperties/index.test.ts	Adds unit tests for custom-property tree-shaking behavior.
ws-nextjs-app/pages/_document.page.tsx	Inlines a regex-based optimiseCssPrefixes implementation in the document rendering path.