This is a good reminder for myself to get some onion addresses for my sites and spread awareness of Tor.
TIL that Onion-Location is a header, only new about the <meta> element.
<meta http-equiv="onion-location" content="http://<your-onion-service-address>.onion" />
The "http-equiv" in that meta tag means "equivalent HTTP header", FYI.
But it's not always followed. Most HTTP headers aren't interpreted when specified with http-equiv, and vide versa.