We should build webkitgtk4 with support for subtle crypto . All we need to do is add a BuildRequires for gnutls-devel.
Blocking FE-Legal out of caution. I'm not aware of any problems and I believe our Firefox package already implements the same features, but I've done no research.
After further discussion upstream, we decided to disable subtle crypto by default on all distros, since the feature is not fully complete.