HTML declares a markdown alternate
An agent reading the HTML page should be able to discover the markdown mirror without guessing the URL. The standard way to declare it is `<link rel="alternate" type="text/markdown">`.
How the check decides
The check queries link[rel="alternate"][type="text/markdown"] on the page and asserts a non-empty href attribute is present. Fails if no such tag exists.
How to implement it
Emit a <link rel="alternate" type="text/markdown"> in your page’s <head> pointing at the markdown mirror you publish.
Pass
<head>
<link rel="alternate" type="text/markdown" href="/docs/install.md">
</head>
Fail
<head>
<!-- no alternate link -->
</head>
Common gotchas
The href can be relative (/docs/install.md) or absolute (https://example.com/docs/install.md); both satisfy the check. Use whichever your site already prefers for canonical and og links — consistency matters more than the specific form.
Make the alternate-link emission part of your shared layout component so it’s set automatically on every page. The alternate’s URL should be derived from the current page’s URL, not duplicated by hand — otherwise you’ll forget to update it when adding a new page or moving an existing one.
The companion check markdown.mirror-suffix verifies the mirror file actually exists at the URL the alternate-link advertises, so adding the link without publishing the mirror still leaves you with a half-fix. Build your mirror generation and your alternate-link emission together.