For WXS validators that will follow a URI to an arbitrary online location, there is the potential for reading something malicious from the other side of the stream.
W3C XML Schema does not implement most of the DTD ability to provide data elements to a document.
XSD schemas are conventionally written as XML documents, so familiar editing and transformation tools can be used.
As well as validation, XSD allows XML instances to be annotated with type information (the Post-Schema-Validation Infoset (PSVI)) which is designed to make manipulation of the XML instance easier in application programs.
There are languages developed specifically to express XML schemas.
The Document Type Definition (DTD) language, which is native to the XML specification, is a schema language that is of relatively limited capability, but that also has other uses in XML aside from the expression of schemas.
Again, this design is intentional and is to separate validation and augmentation.Two more expressive XML schema languages in widespread use are XML Schema (with a capital S) and RELAX NG.The mechanism for associating an XML document with a schema varies according to the schema language.There are several different languages available for specifying an XML schema. The primary purpose of a schema language is to specify what the structure of an XML document can be.This means which elements can reside in which other elements, which attributes are and are not legal to have on a particular element, and so forth.