The JavaScript language is extremely standardized—the differences between Safari, Chrome, and Firefox are almost 100% down to browser APIs like Bluetooth and MIDI support and such. Those things don't come up in the JavaScript engine.

The only exception I can think of is proper tail calls, which Google explicitly removed and I don't know if Mozilla ever tried to implement: https://compat-table.github.io/compat-table/es6/#test-proper... https://chromestatus.com/feature/5516876633341952

There is a language spec, but the browsers implement that standard at different rates. I know up until recently negative look-behind didn't work in Safari but did work in Chrome, and RegEx is clearly a language feature rather than an API.