Complete the HTML and CSS rendering roadmap

[mh0pe]

Summary

• add a dedicated <hr> renderer for computed backgrounds and all four border sides, including dotted, dashed, double, inset/outset, and two-band groove/ridge styles
• render underline, overline, and line-through from their originating decorating box with stable OpenType metrics, multiline geometry, propagation boundaries, and the required below-text/above-text paint order
• preserve complete nested 2D affine transforms, including skew and 2D-compatible matrix3d() values, while rejecting true 3D transforms explicitly
• keep transformed containing blocks stable during geometry measurement
• add regression tests, documentation, and live examples for the completed roadmap

Validation

• yarn install --frozen-lockfile
• yarn test
• focused ESLint over every changed source and test file
• yarn build
• yarn example:build
• git diff --check
• headless Chrome geometry smoke: four-sided and double rules, continuous mixed-size ancestor underline (0 px width delta), 3-below/2-above decoration layer order, preserved skew coefficients, and 0 px drift for a fixed child under a transformed parent

The repository-wide eslint src command still reports the existing Image/backgroundSize findings in src/renderers/div.js; this PR does not expand into those unrelated baseline issues.

Scope notes

The root element's own transform, independent translate/rotate/scale properties, and true 3D or perspective transforms remain outside the supported surface and are now documented explicitly.

I also evaluated chenglou/pretext as a possible backend. It is useful for line breaking and measurement, but it does not provide DOM box geometry, baselines, glyph placement, transforms, or SVG paths. Replacing the current Range/OpenType pipeline with it would reduce fidelity; a future opt-in adapter should first introduce a pluggable layout seam, strict fallback rules, and differential browser tests.