Refer to Genode Handbook for a decent definition and examples.
seL4 is formally proven to enforce capabilities.
Genode is, and should be reasonably easy to test on a regular PC, via bootable usb stick.
It can be quite confusing to use at first, as capabilities have GUI representation.
There's a blog[0] and some introductory video[1].
[0]: https://genodians.org/
[1]: https://youtu.be/vmgWgzeKAjU
Refer to Genode Handbook for a decent definition and examples.
seL4 is formally proven to enforce capabilities.