XHTML+USDM definition to facilitate content validation

[ASL-rmarshall]

The USDM IG recommends that "a single root <div xmlns="http://www.w3.org/1999/xhtml"> element is used to wrap the content of the attribute". This implies that content would be considered valid if it conforms with the XHTML v1.0 Document Type Definition (DTD) found at http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd. However, while the USDM IG says that XHTML fields may include references to USDM elements (usdm:ref and usdm:tag), these references are not allowed according to the XHTML v1.0 DTD and would fail validation.

It would be helpful to have a published DTD (available via a URL) that includes both:

• The XHTML specification that's expected to be used for USDM content and
• The "official" definitions of the USDM elements/attributes.