" id="w3c-conventions"><span class="content">Document conventions</span><a class="self-link" href="#w3c-conventions"></a></h3>
   <p>Conformance requirements are expressed
    with a combination of descriptive assertions
    and RFC 2119 terminology.
    The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL”
    in the normative parts of this document
    are to be interpreted as described in RF