Elements with children having only one occurrence can be defined. The element name is used to declare that the child element can occur only once inside the element.
<?xml version=?1.0? standalone=?yes??> <!DOCTYPE rootelement [ <!ELEMENT rootelement (childelement)> <!ELEMENT childelement (#PCDATA)> ]> <rootelement> <childelement>Child element.</childelement> </rootelement>