The spec currently says "The resulting URL is used as the principal and the host is extracted from it according to URI [RFC3986] syntax rules". About this, Yaron Goland wrote:
Um… no. There is no generic way to find a host in a URI scheme. Yes, section 3.2.2 defines a HOST BNF but it doesn’t control where it is used. You can’t process URLs in general, only specific ones you support. So again, just limit to HTTPS and use the HTTPS parsing rules to get the host.