rather new feature. Please report breakages. This also requires you
to run at least WebKitGTK 2.26 (released 2019-09-09). See also:
https://blogs.gnome.org/mcatanzaro/2020/03/31/sandboxing-webkitgtk-apps/
+ https://www.uninformativ.de/blog/postings/2020-05-04/0/POSTING-en.html
[Added]
- Allow webkit:// in URLs.