The initial draft of the Ecma standard for the Open XML Formats is available at this link: http://www.ecma-international.org/activities/Office%20Open%20XML%20Formats/TC45_FD_XML_docform.zip
It's a massive document, nearly 2000 pages total, which contains all of the reference information about the tags, components, namespaces and other details that make up the proposed Open XML Formats standard.