If none of the conditions described in Sections 3.1 -- 3.3 apply, then the base URL is considered to be the empty string and all embedded URLs within that document are assumed to be absolute URLs. It is the responsibility of the distributor(s) of a document containing relative URLs to ensure that the base URL for that document can be established. It must be emphasized that relative URLs cannot be used reliably in situations where the document's base URL is not well-defined.