According to https://www.rfc-editor.org/rfc/rfc3986 and http://en.wikipedia.org/wiki/Uniform_resource_identifier, a URI may or may not contain a double slash following the scheme identifier. This makes "urn:issn:1535-3613" a valid URI just as "http://stackoverflow.com".
Is there a strict/formal need to include the double slash or is it optional and in any case, what is the reason/semantics? When answering, please provide a conclusive answer - Don't just report how you browser/library/... handles it.
It's in the RFC you linked: If there is a //, it means that what follows that is the authority. See Section 3. So if the scheme uses an authority, it will use the // after the colon (either requiring it, if authority is required in that scheme, or having it be optional if authority is optional in that scheme). mailto doesn't use an authority in the URI sense, so mailto URIs don't include a //.