Some good time zone news: CLDR just merged a PR (https://github.com/unicode-org/cldr/pull/3754) to align CLDR's list of time zone IDs with IANA and (following IANA upstream) deprecates old-school POSIX time zone IDs like PST8PDT and WET. See https://unicode-org.atlassian.net/browse/CLDR-17111 for more details.

After the next CLDR release is integrated into V8 & JSC, it will fix one of the biggest sources of cross-engine divergence in time zone behavior.

Another remaining difference across engines is where ~10 IDs canonicalize differently, e.g. Asia/Calcutta vs. Asia/Kolkata. This will be resolved when V8 & JSC switch to using the new ICU API getIANAId. V8 is planning to switch to this API as part of releasing Temporal Stage 4 unflagged. Not sure about JSC.

The final difference between engines is the list of IDs returned by Intl.supportedValuesOf('timeZone'). SpiderMonkey returns all IDs, while V8 omits Etc/GMT* IDs and JSC omits those as well as UTC. I'd like to take another crack at this one after the two issues above are resolved. See https://github.com/tc39/ecma402/issues/778.