Harness job: Playwright rendered checks against this build's own docutils XML, published under /_harness/

Layer 3 of the theme-parity testing model (QuantEcon/project-theme-parity#13), the Sphinx side. Decided 2026-09-17 (mmcky): the Playwright runs live in the test repositories, each against its own fresh build; QuantEcon/project-theme-parity#15 pairs and compares what they publish.

What this repository adds

The build workflow stays as it is: the production lecture repositories' composite actions (quantecon/actions@v0), in the build image pinned by digest that both corpora share (D14, which amends D4 for the execution environment). A separate job runs after it:

1. Engine output: jb build --builder custom --custom-builder xml alongside the HTML build; the docutils XML for every page is published in the Pages artifact under /_xml/.
2. tests/harness/: a Playwright project modelled on quantecon-book-theme/tests/visual, serving the built _build/html statically. Its locator file maps every family id in project-theme-parity's bin/families.yml to this theme's DOM (div.cell_input, div.cell_output, div.admonition.note, div.proof.theorem, div.math, figure, mjx-container, …) and to the site features (launch buttons, downloads, dark mode, sidebar and in-page TOC, prev/next, search, footer, code copy, output scroll and collapse, footnote and citation hovers, mobile nav, keyboard nav, print, MathJax assistive MathML).
3. Checks, the contract shared with the mystmd repository: drawn (rendered count equals the XML count for the family on the page), text present (numbering and captions), behaviour (toggles, anchors, copy, launch targets, dark mode persistence, TOC, prev/next), error markers (mjx-merror, .system-message, .problematic), console clean; at desktop 1280, tablet 820 and phone 390, light and dark.
4. Metrics and crops: per family and page, computed font size and line height relative to body, colour, background, WCAG contrast, width relative to the column; an element-level PNG per family, page, viewport and scheme, named by family id.
5. Publication: _harness/rendered.csv, _harness/metrics.csv, _harness/crops/… in the Pages artifact at every push to main, so a pass can cite them by URL and commit.

Exit

/_xml/ and /_harness/ are live on the site; the job is green; a drawn failure on any page fails the job. Coordinated with the mystmd repository's twin item so both publish the same CSV schema.

Start with the existing build workflow and tests/harness/visual model, then read project-theme-parity's bin/families.yml and the coordinated mystmd item. Run the jb XML build and Playwright checks against _build/html at the three stated viewports and both schemes. Done means /_xml/ and /_harness/ are published, the job is green, and rendered.csv and metrics.csv are available with a drawn failure causing failure.