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"HTML 4.0 has about three hundred different tags. Most of
these have half a dozen possible attributes for several thousand
variations. Because XML is more powerful than HTML, you may think XML
would have even more tags, but you'd be wrong....XML predefines almost no
tags." - Elliotte Rusty Harold
As we have said before, XML is a tool used to generate markup languages in
general rather than a specific markup language. Thus, rather than
pre-defining a set of tags, XML defines a methodology for tag creation.
Once defined, tags are mixed with plain text in order to form an "XML
document".
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It is worth mentioning that the word "document" can be a little misleading because although XML markup can certainly be contained in a file, (as the word document would imply), it can also be sent as a data stream, a database result set, or be dynamically-generated by one application and sent to another. More correctly, an XML document can be thought of as a "data object", but for simplicity, document will work just fine.
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However, though you are free to be as innovative as you want with the tag sets you create, you must follow the constraints of the XML tag set generation standards exactly. When an XML document is presented to an XML-processor, in order for the XML processor to understand how to process it, the XML must follow the XML standard. Specifically, the document must be "well-formed". If the document is not well-formed the processor will stop, complaining about a "fatal error".
Well-formedness has an exact meaning in XML. Specifically, a well-formed
document adheres to the syntax rules specified by the XML 1.0 specification
in that it must satisfy both physical and logical structures.
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Why get so caught up in syntax? Well, the creators of XML had a tough problem to solve.
They had to create a system in which documents could be created that could be read either
by people or by machines. Writing a language for people is one thing...people can
figure their way through ambiguity. Machines, on the other hand, can only work if
the rules are clear and the rules are followed. Making your XML document well-formed means that it
meets the minimum requirement of being readable by the dumbest of computers.
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