HTML and CSS Reference
In-Depth Information
The example document begins with the optional <?xml> directive declar-
ing the version of XML it uses. It then uses the <!DOCTYPE> directive to
identify the DTD that some automated system, such as a browser, uses
to process and perhaps display the contents of the document. In this
case, a DTD named blah.dtd should be accessible to the browser [*] so
that the browser can determine whether the <blah> tag is valid within
the document.
[*] We use the word browser here because that's what most people will use to process and view XML
documents. The XML specification uses the more generic phrase "processing application" because, in
some cases, the XML document will be processed not by a traditional browser, but by some other tool
that knows how to interpret XML documents.
XML DTDs contain only XML entity and element declarations. XML docu-
ments, on the other hand, may contain both XML element declarations
and conventional content that uses those elements to create a docu-
ment. This intermingling of content and declarations is perfectly accept-
able to a computer processing an XML document, but it can get con-
fusing for humans trying to learn about XML. For this reason, we focus
our attention in this chapter on the XML entity and element declaration
features that you can use to define new tags and document types. In
other words, we are addressing only the DTD features of XML; the con-
tent features mirror the rules and requirements you already know and
use in order to create HTML documents.
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