Why do UTC offset and timezone look inconsistent?

The IANA timezone is a rule-set; the UTC offset is computed right now. During DST, the same timezone has a different offset than in winter.

Why does 'preferred languages' differ from my OS language?

Browsers let you configure a separate language preference list, independent of the OS. Servers use this list in the Accept-Language header.

It relies on Intl.Locale.getWeekInfo() / weekInfo, which is only implemented in newer browsers. Older builds do not expose it.

Reading locale and timezone…

See every value your browser exposes about time, date, locale, and language — live, via the Intl API.