for me it's just a slight reduction in the font size due to firefox ignoring the above - which doesn't seem enough of a difference to warrant folk calling it outright "broken"

It's entirely possible I'm misreading the spec on clamp() but as far as I understood it, clamp() performs math on a bunch of comma separated fixed values, calc() handles arithmetic operators - this seems to be combining those without a calc invocation, but maybe I misread & overlooked clamp arithmetic operators. Firefox just says it's invalid, no detail on why.