This was a browser feature that was relatively unknown until recently when several browsers announced that they would be removing support. It is useful to track instances when a user clicks on a link, as it could be recorded by the server for analysis of user activity. As it was done out-of-bounds, there was no additional code, such as javascript, required on the client-side for this to occur.
<a href="..." ping="https://example.com/pingreporter">Example link</a>
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