Sources

w3schools.com tutorials

• http://www.w3schools.com/xml/
• http://www.w3schools.com/dtd/
• http://www.w3schools.com/schema/

• http://www.w3schools.com/xsl/
• http://www.w3schools.com/xslfo/
• http://www.w3schools.com/xpath/

• http://www.w3schools.com/dom/

Wikipedia

• http://en.wikipedia.org/wiki/WSDL
  

Sun tutorials

• http://java.sun.com/webservices/docs/ea2/tutorial/doc/IntroXML3.html - XML and Related Terms
• http://java.sun.com/webservices/docs/ea2/tutorial/doc/JAXPXSLT.html - XSLT and XPath

http://www.w3.org/TR/REC-xml/ - XML
http://www.xmlfiles.com/xsl/ - XSL Tutorial

http://relaxng.org/ - RELAX NG
http://relaxng.org/tutorial-20011203.html - RELAX NG Tutorial

http://www.xml.com/pub/a/2000/11/29/schemas/part1.html - Using W3C XML Schema
http://www.w3.org/TR/xmlschema-0/ - XML Schema Tutorial
http://www.xfront.com/DefaultNamespace.html - XML Schemas: Best Practice

http://pear.php.net/manual/en/standards.php - PHP Manual

http://www.garshol.priv.no/download/xmltools/cat_ix.html - XML Tools by Category
http://www.garshol.priv.no/download/xmltools/plat_ix.html - XML Tools by Platform