[itsnotlupus]

Chrome sets it if you tell it you want to use its spellchecker with some languages.

I'm not sure what the standard says, but i suspect you'd surprise users considerably less if you used the first language listed in that header rather than the last (assuming equal or missing q values).

[mholt]

It's been years since I looked at the code/config but I literally think it's a "strings.Contains()" call... so not even parsing at all.

A proper implementation would make for a good Caddy module.