[ShadowBanThis01]

Thinking it through, I assume it's on the client side. Maybe something is wrong with the JavaScript that's doing the validation... but do Safari and Firefox use the same engine?

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.