Venture pages are written against a placeholder base URL, a reserved .invalid host, and build_pages_site.py rewrites it to the host that actually deploys. In the source tree that is 468 canonical links, 150 feed links and a few cover images.
The rewrite was never checked. If it missed one, a canonical or a feed would publish pointing at a host that does not resolve, and nothing would fail.
platform/ci/check_built_site.py now scans the built _site for the placeholder and runs in pages.yml after the link check. Today's build of 1,319 products (12,907 files) has none left. localhost is deliberately not flagged: product READMEs quote it as run-locally instructions, and a first draft that flagged it reported 90 false hits.
Two nearby sweeps came back clean and need no follow-up: no venture page loads a script or image over plain http://, and the only third-party hosts any page loads from are Google Fonts (one page).