Safari not rendering javascript or css on some pages in MacOS. Works fine on another Mac and iPad

For whatever reason, on a new M1 iMac, some webpages won't render correctly, like javascript and/or css is turned off. No graphics. just text. Where a graphic should be it's just a little box with a "?" in the middle. This only does it on the one Mac. It also does it using Firefox on the same iMac. When I try it on a different M1 Mac Mini, it works fine, and it also works fine on an iPad.

I've rebooted the iMac into recovery and used Safari there, and the page works, but logged in, it stops rendering the page correctly again. I've cleared the cache, I've disabled plug-ins. But again, the page works fine on a different Mac WITH the plug-ins installed and working.

I could kind of understand if this was just happening to Safari, but on a totally different browser like Firefox too?