A header as it was typed, or null where that is not a header at all.

Both spellings, because both hands type both: accept=application/json is what you write when you are assigning something, and accept: application/json is what you get when you copy a line out of a response. Whichever separator comes FIRST is the separator, which is what makes content-type: text/html; charset=utf-8 land the way it reads rather than splitting on the charset=.

The name is lower-cased because HTTP does not distinguish, and a session that thought Accept and accept were two headers would send both.

import { parseHeader } from './http-service.ts';

parseHeader('Accept=application/json'); // { name: 'accept', value: 'application/json' }
parseHeader('content-type: text/html; charset=utf-8')?.value; // 'text/html; charset=utf-8'
parseHeader('accept'); // null — nothing assigned
parseHeader('bad header=x'); // null — a space is not allowed in a header name