[sfink]

JS is specced up the wazoo, including the exact execution semantics. It's fairly difficult to write JS that executes differently between Chrome & Firefox. The only things with intentional nondeterminism in the core language are WeakRef and FinalizationRegistry. And those won't just behave differently between browsers, they may behave differently across browser versions, loads, user activity, etc.

Date parsing has different edge cases between browsers.

The DOM obviously provides more nondeterminism as well as deterministically different behavior, and there are some different things exposed to web pages so it's not like you can't detect which engine you're running on. But things like exact switch case execution ordering are well-covered by the spec.