fn check_timezone(value: &Value) -> ResultExpand description
Validate an MSC4175 time zone against the shape of an IANA Time Zone Database name.
Membership in a bundled copy of the database is deliberately not tested: the proposal’s rationale for a loose check is that clients and servers carry different database versions, so a browser one release ahead of ours would have a newly added zone rejected. Shape alone still refuses offsets, platform display names, and anything else that could never name a zone.