Maybe not an HTTP client but a modern browser with its JavaScript engine, encryption support, CSS engine, etc I think for most people really is a lot of work. If one does rely on the right libraries it's a weekend of fun (and pain) so I would argue between few hours and a lifetime, depending on how much you implement.

My point is that the essence, the core mechanism, is typically a 1-liner, but only with a pedagogical context, not a production (in the sense of anybody else using it reliably in realistic situations) context.