I like neocities.org for small experiments like this but you can use anything you’d like.
I don't know what else could vary based on platform. One thing I can't control for is browser and OS versions, because the older Mac I have (the one that works in this case) can't be updated to the latest.
It must be sending garbage characters. But the vast, vast majority of sites work. It's really mystifying.
As far as other sites are concerned... I just remembered one that's not quite as clear-cut. Zoro.com complains that my credentials are wrong, but if I reset my password it complains that I'm trying to use my current password... so it's obviously not wrong. As usual, they just threw up their hands... and sent me a 10% coupon... which of course I can't use because their site is broken. Just tried it again, and it's still unusable. Didn't try the older computer yet.
On the other hand, I find a lot of bugs. Not in things as refined as compilers, usually, but if I had a dollar for every time I've heard, "Well, nobody else has reported this before," I could probably buy a tank of gas. And that's saying something today.
Latest example: Amazon's Web site rejects all phone numbers entered on an Apple Silicon Mac. I was setting up a new address, and it said "remove invalid characters from phone number field:" https://i.imgur.com/mjwiCqc.png
This happened in both Safari and Firefox. Amazon support couldn't figure it out in 1.5 hours of chat. I went to an Intel Mac and entered the exact same number, no problem.