Xfinity.com doesn't display navigation in any browser on Mac OS?

There are some websites that don't work well in Safari, but I've run into an unusual problem with Xfinity.com (I have the misfortune of being an Xfinity customer, but perhaps not for much longer).

Xfinity has revamped their web presence and their new site does not display any top navigation whatsoever, in any browser on Mac OS, that might permit login. I've tried Safari 17.3, Firefox (current version), Chrome (current version), repeatedly cleared cookies and cache. Xfinity support has given me alternative URLs, but I still can't log in. Even Xfinity speed test -- at an Xfinity URL -- will not run in a Mac OS browser.

The main nav with login is supposed to be at the top of Xfinity.com, but all I get is the solid black bar (when the site isn't too busy to time out):

I cannot log in when using my desktop. I can log in if I open Xfinity.com on my phone very much not ideal -- I need to use the desktop so I can clean out hundreds of emails from 6 email accounts.

Is it possible that Xfinity's web developers managed to make a website that's not compatible with desktop Mac OS, in any Mac OS browser? I wouldn't put it past Xfinity.