The following web sites provide useful XML information:

You might find the following XML-related resources interesting, even though they are bit theoretical:

  • wrote an article entitled The Difficulty of Schema Conformance Problems; it was published on .

    The article examines the difficulty of the schema conformance problem for the following XML schema languages:

    • DTD
    • RELAX NG with W3C XML Schema Datatypes
    • Schematron
    • W3C XML Schema
    • Namespace Routing Language

    The schema conformance problem is defined as follows: Given a schema, is there an XML document that conforms to the schema?

    For these five XML schema languages, the schema conformance problem is intractable (ranging from NP-hard to undecidable).

    There is also an earlier version of the article.

  • The TMML Home Page, which describes the Turing Machine Markup Language (TMML) and a TMML interpreter written in XSLT.