Sun has released a tool that abstracts validation to its core, and builds different validation syntaxes on top.
This allows the same engine to work with DTD, XML Schema, and even Relax/NG.
The XML Schema Working Group is closed, having completed its work successfully.
The specifications are now maintained by the W3C XML Core Working Group.
You only need the second for this to work, but the first contains the documentation that you might want to peruse at a later time.
In case you are having difficulties downloading them, here are archived copies of msv*and relnames*
See also: Robin Cover's index of XML Schema materials.
XML Schema 1.1 (in two parts) is a W3C Recommendation.
Since each validator implements some slight variation of the standard, it's often useful to check schemas against more than one validator to catch problems that one might miss.Open any XML file with an attached schema, and click on the (Validate Document) icon to drop down the list, and then select the Multi Schema Validator entry.