W3 Schools    http://www.w3schools.com/default.asp
    - DTD          http://www.w3schools.com/dtd/dtd_intro.asp
    - XML Schema http://www.w3schools.com/schema/schema_intro.asp
    - XSLT      http://www.w3schools.com/xml/xml_xsl.asp
    - XPath     http://www.w3schools.com/xpath/default.asp
    - DOM       http://www.w3schools.com/dom/default.asp
    - XQuery   http://www.w3schools.com/xquery/default.asp