Markdown content negotiation

What it checks

The extension requests the page being scanned (not the homepage) with the Accept header a real agent sends:

Accept: text/markdown, text/html;q=0.9, */*;q=0.8

It passes when the response is successful, has a text/markdown (or text/x-markdown) content type, and the body does not start like an HTML document. An error page labelled as Markdown does not count.

If that fails, the extension tries two fallbacks so it can tell you what is close to working:

• A Markdown-looking body served as text/plain, which differs from what the same URL returns for Accept: text/html.
• Markdown served only when the request sends a bare Accept: text/markdown, which no real client sends.

This is the heaviest check in Content accessibility. The .md URL and rel="alternate" routes to Markdown are separate checks: Markdown URL fallback and Markdown alternate link.

Results

Status	When
Pass	The realistic Accept header gets a successful text/markdown response
Warn	A Markdown body is returned, but labelled text/plain
Warn	Markdown is returned only for a bare Accept: text/markdown, not the realistic header
Fail	No Markdown version is offered through content negotiation

How to fix

When a request’s Accept header prefers text/markdown, respond with a Markdown version of the page and Content-Type: text/markdown. Parse the header properly, including quality values, rather than matching the exact string text/markdown.

Also send Vary: Accept, so caches keep the HTML and Markdown responses apart (see Markdown negotiation caching).

To test it:

curl -sI -H 'Accept: text/markdown, text/html;q=0.9, */*;q=0.8' https://example.com/page

The response should include Content-Type: text/markdown.