The document of a DTD is as follows:
The definition of the element element, # pcdata (Parsable Character Data) represents the text that XML can interpret.
"*, ,?" is quantum symbol, * indicates that there are 0 and infinite ,? Indicates that there can only be 0 or one, indicate at least one,
Attlist attribute declaration, the element can only use the properties here, "|" or means, and the last RMB is the default attribute.
Mount DTD document
