THE XML REVOLUTION - TECHNOLOGIES FOR THE FUTURE WEB

SGML relics

- only a fool does not fear "external general parsed entities"

As an unfortunate heritage from SGML, the header of an XML document may contain a document type declaration:
<?xml version="1.0"?> <!DOCTYPE greeting [ <!ELEMENT greeting (#PCDATA)> <!ATTLIST greeting style (big|small) "small"> <!ENTITY hi "Hello"> ]> <greeting> &hi; world! </greeting>

This part can contain:

DTD (Document Type Definition) information:
- element type declarations (ELEMENT)
- attribute-list declarations (ATTLIST)
(described later...)
entity declarations (ENTITY) - a simple macro mechanism
notation declarations (NOTATION) - data format specifications

Avoid all these features whenever possible!

Unfortunately, they cannot always be ignored - all XML processors (even non-validating ones) are required to:

normalize attribute values (prune white-space etc.)
handle internal entity references (e.g. expand &hi; in greeting)
insert default attribute values (e.g. insert style="small" in greeting)

according to the document type declaration, if a such is present.