built-in data type
SGML and XML elements can have different types of content embedded in them. The content-model is
an abstract type denoting the type of content that an element can contain. Every element-declaration
must have a content-model assigned to it.
The following six constants are the only possible values of content-model:
any-content-model
cdata-content-model
element-content-model
empty-content-model
mixed-content-model
rcdata-content-model