Accessibility / content QA: media sweep + html-proofer gate (V2.0)

[jonfroehlich]

Accessibility / content QA (V2.0)

Umbrella for the V2.0 a11y + content-QA work. Two PRs:

PR #112 (merged) — media a11y content sweep

Fixed 103 source-level a11y issues across 16 pages (iframe title=, video aria-label, image alt), written to describe the actual media.

PR #113 — off-the-shelf content-QA gate + baseline triage

• Adopt html-proofer (link-check job) for broken links/anchors/missing-alt/HTML — industry-standard, no new toolchain (Ruby). External checks disabled (deferrable to a separate non-blocking job).
• Keep a slimmed check_a11y (media-a11y job) for the source conventions html-proofer can't see (empty alt="", iframe/video labels) — the a11y analogue of check_seo_frontmatter.
• Fix everything the gate surfaced: broken media, a site-wide protocol-relative aux link, and 13 broken internal links/anchors.
• Document both gates + local-run/libcurl/WSL2 notes in website-dev.md.

Decision

Use industry-standard tooling for commodity problems (links/HTML → html-proofer); reserve small bespoke checks (check_a11y, check_seo_frontmatter) only for conventions with no off-the-shelf equivalent. Chose html-proofer over pa11y/axe to avoid a Node + Chromium toolchain; deep WCAG (contrast/ARIA) via axe/pa11y remains an optional later layer.

Tracked debt (gate ignores, each annotated in content-lint.yml)

• ESP32 capacitive-touch: add 4 missing circuit diagrams #114 — ESP32 capacitive-touch: add 4 missing circuit diagrams (WIP)
• html-proofer baseline debt: accel.md screenshot + signals notebook alt #115 — accel.md screenshot + stale signals nbconvert notebooks (notebooks moving to makeabilitylab/signals, Signals Lessons #116)

Out of scope

• Replace color-only wiring/circuit instructions with redundant cues (a11y, WCAG 1.4.1) #111 — replace color-only wiring instructions (WCAG 1.4.1; manual pass)

Closed by #113 (residual content debt tracked in #114/#115).