Learning XML

2.7. Well-Formed Documents

XML gives you considerable power to choose your own element types and invent your own grammars to create custom-made markup languages. But this flexibility can be dangerous for XML parsers if they don't have some minimal rules to protect them. A parser dedicated to a single markup language such as an HTML browser can accept some sloppiness in markup, because the set of tags is small and there isn't much complexity in a web page. Since XML processors have to be prepared for any kind of markup language, a set of ground rules is necessary.

These rules are very simple syntax constraints. All tags must use the proper delimiters; an end tag must follow a start tag; elements can't overlap; and so on. Documents that satisfy these rules are said to be well-formed. Some of these rules are listed here.

The first rule is that an element containing text or elements must have start and end tags.

Good	Bad
<list> <listitem>soupcan</listitem> <listitem>alligator</listitem> <listitem>tree</listitem> </list>	<list> <listitem>soupcan <listitem>alligator <listitem>tree </list>

An empty element's tag must have a slash (/) before the end bracket.

Good	Bad
<graphic filename="icon.png"/>	<graphic filename="icon.png">

All attribute values must be in quotes.

Good	Bad
<figure filename="icon.png"/>	<figure filename=icon.png/>

Elements may not overlap.

Good	Bad
<a>A good <b>nesting</b> example.</a>	<a>This is <b>a poor</a> nesting scheme.</b>

Isolated markup characters may not appear ...