RDDL contains elements from the XHTML and RDDL namespaces.
Rules defining occurrence rules for the RDDL resource element
and its attributes. Note that for attributes, it is not that the attribute is being tested for existance,
but whether it has a value.
RDDL resource elements must be within the html body.
When lang attribute is present, the value must equal the value of xml:lang.
RDDL resources must be within the html body.
There should be at least one RDDL resource in the body.
The element rddl:resource should contain only the attributes id, xml:base, xml:lang, xlink:href, xlink:arcrole, xlink:role, xlink:href.
The rddl:resource element should only have child elements from the XHTML and RDDL namespaces.
RDDL resources are simple Xlinks.
xlink:embed,xlink:show are not used.
The value of xlink:role cannot be a relative URI reference.
The value of xlink:arcrole cannot be a relative URI reference.
A rddl:resource should have a description.