side of the stream. W3C XML Schema does not implement most of the DTD ability to provide data elements to a document. Although W3C XML Schema's ability to add default attributes to elements is an advantage, it is a disadvantage in some ways as well. It means that an XML file may not be usable in the absence of its schema, even if the document would validate against that schema. In effect, all users of such an XML document must also implement the W3C XML Schema specification, thus ruling out minimalist or older XML parsers. It can also slow down the processing of the document, as the processor must potentially download and process a second XML file (the schema); however, a schema would normally then be cached, so the cost comes only on the first use. Tool Support WXS support exists in a number of large XML parsing packages. Xerces and the .NET Framework's Base Class Library both provide support for WXS validation. RELAX NG RELAX NG provides for most of the advantages that W3C XML Schema does over DTDs. Advantages over W3C XML Schema While the language of RELAX NG can be written in XML, it also has an equivalent form that is much more like a DTD, but with greater specifying power. This form is known as the compact syntax. Tools can easily convert between these forms with no loss of features or even commenting. Even arbitrary elements specified between RELAX NG XML elements can be converted into the compact form. RELAX NG provides very strong support for unordered content. That is, it allows the schema to state that a sequence of patterns may appear in any order. RELAX NG also allows for non-deterministic content models. What this means is that RELAX NG allows the specification of a sequence like the following: <zeroOrMore> <ref name="odd" /> <ref name="even" /> </zeroOrMore> <optional> <ref name="odd" /> </optional> When the validator encounters something that matches the "odd" pattern, it is unknown whether this is the optional last "odd" reference or simply one in the zeroOrMore sequence without looking ahead at the data. RELAX NG allows this kind of specification. W3C XML Schema requires all of its sequences to be fully deterministic, so mechanisms like the above must be either specified in a different way or omitted altogether. RELAX NG allows attributes to be treated as elements in content models. In particular,