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LINKING AND PRESENTATION STANDARDS
an “address book” rather than directly to the target document—updating the
address book then automatically changes any links that use it).
XML Base
This standard defines an attribute for XML documents that defines a “base”
address, that is used when evaluating a relative address specified in the document. (So, for example, a simple file name would be found in the baseaddress directory.)
XPointer
In general, the XLink specification targets a document or document-segment
using its ID. The XPointer specification defines mechanisms for “addressing
into the internal structures of XML documents”, without requiring the
author of the document to have defined an ID for that segment. To quote the
spec, it provides for “reference to elements, character strings, and other parts
of XML documents, whether or not they bear an explicit ID attribute”.
For more information on the XML Linking standards, see
http://www.w3.org/XML/Linking.
XHTML
The XHTML specification is a way of making XML documents that look and act
like HTML documents. Since an XML document can contain any tags you care
to define, why not define a set of tags that look like HTML? That’s the thinking
behind the XHTML specification, at any rate. The result of this specification is a
document that can be displayed in browsers and also treated as XML data. The
data may not be quite as identifiable as “pure” XML, but it will be a heck of a lot
easier to manipulate than standard HTML, because XML specifies a good deal
more regularity and consistency.
For example, every tag in a well-formed XML document must either have an
end-tag associated with it or it must end in />. So you might see <p>...</p>, or
you might see <p/>, but you will never see <p> standing by itself. The upshot of
that requirement is that you never have to program for the weird kinds of cases
you see in HTML where, for example, a <dt> tag might be terminated by </DT>,
by another <DT>, by <dd>, or by </dl>. That makes it a lot easier to write code!
The XHTML specification is a reformulation of HTML 4.0 into XML. The latest
information is at http://www.w3.org/TR/xhtml1.
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