At present, in rust-http, invalid values will be dropped; that will be changing so that you can retrieve a bad header value (https://github.com/chris-morgan/rust-http/issues/27). Writing invalid HTTP, on the other hand, is something that I'm in two minds about. You shouldn't be allowed to do it, but it may be necessary somewhere along the way. At present, you can write invalid headers through the use of the extension headers—`response.headers.extension.set(~"Date", ~"invalid")`, for example. I certainly don't want to advertise it broadly, but I'm thinking guaranteeing that that will always work is perhaps the best plan.