Yeah, it's a mess.

Found [another discussion with Fielding](http://w3-org.9356.n7.nabble.com/Host-header-with-an-empty-value-td249428.html) on this issue, where he shows this example:

    GET urn:ietf:rfc:2616.txt HTTP/1.1 
    Host:

So maybe the `must carry an empty Host` sentence from the spec actually refers to requests with URIs whose URI *type* does not allow for an authority component. This would make more sense.

And of course there are also request with a `*` target like this one:

    OPTIONS * HTTP/1.1

I guess they too require a (really) empty `Host` header (without port).

Anyway.
It's a mess...