Question: What does Intl.DateTimeFormat() - "accepts IANA timezone names" actually mean?

[tmtron]

When Intl.DateTimeFormat() - accepts IANA timezone names is green, then does this mean:

1. that the browser has full IANA timezones support? i.e. we can pass any IANA timezone name and get the correct result for that timezone?
2. or does it only mean, that we can pass valid IANA timezone names, but the browser implementation may fall-back to UTC?

It would be great if this could be clarified in the table somehow: i.e. maybe as an info tooltip.

Here's a related quote from the MDN docs:

timeZone
The time zone to use. The only value implementations must recognize is "UTC"; the default is the runtime's default time zone. Implementations may also recognize the time zone names of the IANA time zone database, ...

and a related SO question: How to ensure/load locales on the client?