Every element used in a valid document must be declared in the document’s document type definition (DTD). Element declarations specify the markup tags that can appear in a conforming document and how those tags can be used. If a DTD doesn’t declare elements that are used in a document, the document can’t be said to be valid.
As you read this chapter, keep in mind that XML is case-sensitive. This means you must use the exact case for keywords and declarations specified. There are no exceptions.
Five types of elements are used in XML documents:
Those with standard content
Those with only character data
Those with mixed content
Those with any type of content allowed
Those with no content allowed ...