Server returns markdown for Accept text/markdown
Some agents prefer to ask for markdown with an Accept header rather than guess a URL. Honoring Accept lets a single canonical URL serve both HTML and markdown depending on who's asking, without forcing the agent to know the .md mirror convention.
How the check decides
The check refetches the page URL with Accept: text/markdown and inspects the response’s Content-Type. Passes if it contains text/markdown or text/x-markdown. Fails (with the actual content type) otherwise. Returns error if the request itself fails.
How to implement it
Add a content-negotiation layer in your edge or origin server: when Accept includes text/markdown, serve the .md mirror’s body with Content-Type: text/markdown; charset=utf-8. On Cloudflare Workers / Vercel Edge / Netlify Edge this is ~10 lines of code.
Pass
GET /docs/install
Accept: text/markdown
HTTP/1.1 200 OK
Content-Type: text/markdown; charset=utf-8
# Install a14y
...
Fail
GET /docs/install
Accept: text/markdown
HTTP/1.1 200 OK
Content-Type: text/html; charset=utf-8
<!doctype html>...