Continuing our trip through the basics of XML, there are several rules you need to keep in mind when creating XML documents. All stylesheets we develop in this book are themselves XML documents, so all the rules of XML documents apply to everything we do. The rules are pretty simple, even though the vast majority of HTML documents don’t follow them.
One important point: the XML 1.0 specification makes it clear that when an XML parser finds an XML document that breaks the rules, the parser is supposed to throw an exception and stop. The parser is not allowed to guess what the document structure should actually be. This specification avoids recreating the HTML world, where lots of ugly documents are still rendered by the average browser.
The first element in your XML document must contain the entire document. That first element is called the document element or the root element. If more than one document element is in the document, the XML parser throws an exception. This XML document is perfectly legal:
<?xml version="1.0"?> <greeting> Hello, World! </greeting>
To be precise, this document is well-formed. XML documents are described as well-formed and valid (we’ll define those terms in a minute). This XML document isn’t legal at all:
<?xml version="1.0"?> <greeting> Hello, World! </greeting> <greeting> Hey, Y'all! </greeting>
There are two root elements in this document, so an XML parser refuses to process it. Also, ...