Tighten HTML5 validation in site-health CI

[jmxpearson]

Summary

The Validate HTML step in .github/workflows/site-health.yml is currently continue-on-error: true — it runs and reports HTML5 spec issues to the workflow log but doesn't gate PRs. This was a pragmatic call in PR #32 because the Bootstrap-3-era templates produce a large volume of warnings, and triaging the .html5validator.yaml ignore list was out of scope.

Two paths (pick one)

Option A: Tighten via ignore list

1. Run html5validator --root _site --config .html5validator.yaml locally and dump the output.
2. Categorize errors: which are legitimate Bootstrap-3-era idioms (role="..." on certain elements, deprecated attributes, etc.) vs. real bugs?
3. Add regex patterns to .html5validator.yaml's ignore_re: list for the legitimate ones.
4. Fix the real bugs.
5. Flip continue-on-error: false so future template regressions get caught.

Pros: cheap, immediate signal on contributor changes.
Cons: ignore list will need maintenance.

Option B: Modernize templates

Rewrite _layouts/, _includes/, and hand-written HTML pages (people.html, publications.html) to pass HTML5 strict mode without an ignore list. Pairs naturally with a Bootstrap 3 → 5 upgrade if the lab decides to take that on (the original site-improvement report flagged this as a "low migration risk; afternoon of work" item).

Pros: eliminates the warning class entirely, modernizes the site CSS.
Cons: bigger lift, touches presentation.

Acceptance

Whichever option:

• bundle exec jekyll build && html5validator --root _site --config .html5validator.yaml exits clean locally
• Set continue-on-error: false on the Validate HTML step (or remove the property)
• Push a deliberate HTML error (e.g., unclosed tag) to a branch and confirm the check fails on it