Webmention is a way to tell a url that you have linked to it.
See Webmention Rocks for how to test it.
This post is only really here so you can see webmentions arrive.
This test verifies that you accept a Webmention request that contains a valid source and target URL. To pass this test, your Webmention endpoint must return either HTTP 200, 201 or 202 along with the appropriate headers.
If your endpoint returns HTTP 201, then it MUST also return a Location header. If it returns HTTP 200 or 202, then it MUST NOT include a Location header.
Webmention Rocks!