Re: https://www.w3schools.com/tags/att_a_rel.asp
Re: https://html.spec.whatwg.org/multipage/links.html#other-link-types
Add missing rel=opener value pair to the W3 Schools list, and any others that are missing.
Firefox 79 now requires rel=opener to be explicitly set to enable window.opener use in combination with target="_blank".
https://hacks.mozilla.org/2020/07/firefox-79/