I use this cpcheckme.com site at the end of CCSA to test Autonomous Threat Prevention and also in my Gateway Performance Optimization Class, and it is always a complete crapshoot as to which browsers will actually work with it on Windows systems when passing through a gateway.
In some cases the site has too many redirects as shown above, comes up with "broken" image links and nothing can be clicked, other times it comes up fine and is clickable but when the tests are run it just spins forever and does nothing. Attendees have Chrome, Firefox, Edge and even crusty old IE available and in some cases none of those four browsers will work. Workaround in that case is to disable HTTPS Inspection & reinstall policy which suddenly will make the site work with some (but not all) browsers, which makes no sense since this site is deliberately http-only.
Gateway Performance Optimization R81.20 Course
now available at maxpowerfirewalls.com