This document uses XML DTD fragments ([W3C.REC-xml-20081126], Section 3.2) as a purely notational convention. WebDAV request and response bodies cannot be validated by a DTD due to the specific extensibility rules defined in Section 17 of [RFC4918] and due to the fact that all XML elements defined by that specification use the XML namespace name "DAV:". In particular,
1. element names use the "DAV:" namespace,
2. element ordering is irrelevant unless explicitly stated,
3. extension elements (elements not already defined as valid child elements) can be added anywhere, except when explicitly stated otherwise, and
4. extension attributes (attributes not already defined as valid for this element) can be added anywhere, except when explicitly stated otherwise.
The XML elements specified in this document are defined in the "urn:ietf:params:xml:ns:caldav" XML namespace registered by CalDAV [RFC4791].
When XML element types in the namespaces "DAV:" and "urn:ietf:params:xml:ns:caldav" are referenced in this document outside of the context of an XML fragment, the strings "DAV:" and "CALDAV:" will be prefixed to the element types, respectively.
This document inherits, and sometimes extends, DTD productions from Section 14 of [RFC4918].
Also note that some CalDAV XML element names are identical to WebDAV XML element names, though their namespace differs. Care needs to be taken not to confuse the two sets of names.