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So, this might have you asking the question: “Great — where can I get a listing of XML tags?” Well,
you can’t — at least, not in the sense that you’re thinking when you ask the question. XML has very
few tags that are actually part of the language. Instead, it provides ways of defining your own tags
and utilizing tags defined by others (such as the industry groups I mentioned earlier in the chapter).
XML is largely about flexibility — which includes the ability for you to set your own rules for
your XML through the use of either an XML schema document or the older Document Type Definition
(DTD).
An XML document has very few rules placed on it just because it happens to be XML. The biggie is that
it must be what is called well formed. We’ll look into what well formed means shortly. Now, just because
an XML document meets the criteria of being well formed doesn’t mean that it would be classified as
being valid. Valid XML must not only be well formed, but must also live up to any restrictions placed on
the XML document by XML schemas or DTDs that document references. We will briefly examine DTDs
and XML schemas later on in this chapter.
XML can also be transformed. The short rendition of what this means is that it is relatively easy for you
to turn XML into a completely different XML representation or even a non-XML format. One of the most
common uses for this is to transform XML into HTML for rendering on the Web. The need for this transformation
presents us with our first mini-opportunity to compare and contrast HTML with XML. In the
simplest terms, XML is about information, and HTML is about presentation.
The information stored in XML is denoted through the use of what are called elements and attributes.
Elements are usually created through the use of an opening and a closing tag (there’s an exception, but
we’ll see that later) and are identified with a case-sensitive name (no spaces allowed). Attributes are
items that further describe elements and are embedded in the element’s start tag. Attribute values must
be in matched single or double quotes.
Parts of an XML Document
Well, a few of the names have already flown by, but it makes sense, before we get too deep into things,
to stop and create something of a glossary of terms that we’re going to be utilizing while talking about
XML documents.
What we’re really going to be doing here is providing a listing of all the major parts of an XML document
that you will run into, as shown in Figure 16-1. Many of the parts of the document are optional,
though a few are not. In some cases, having one thing means that you have to have another. In other
cases, the parts of the document are relatively independent of each other.
We will take things in something of a hierarchical approach (things that belong “inside” of something
will be listed after whatever they belong inside of), and where it makes sense, in the order you’ll come
across them in a given XML document.
The Document
Chapter 16: A Brief XML Primer
The document encompasses everything from the very first character to the last. When we refer to an
XML document, we are referring to both the structure and the content of that particular XML document.
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