"This document is a Proposed Edited Recommendation of the W3C. This third edition is not a new version of XML. As a convenience to readers, it incorporates the changes dictated by the accumulated errata...
In addition, markup has been introduced on a significant portion of the prescriptions of the specification, clarifying when prescriptive keywords such as MUST, SHOULD and MAY are used in the formal sense defined in IETF RFC 2119."