An element is valid if there is a declaration matching elementdecl where the Name matches the element type and either the declaration matches EMPTY and the element has no content, or the declaration matches children and the sequence of child elements belongs to the language generated by the regular expression in the content model, with optional whitespace (characters matching the nonterminal S) between the start-tag and the first child element, between child elements, or between the last child element and the end-tag. Note that a CDATA section containing only whitespace does not match the nonterminal S, and hence cannot appear in these positions. The declaration matches Mixed and the content consists of character data and child elements whose types match names in the content model. The declaration matches ANY, and the types of any child elements have been declared.

If a document includes a DTD with element declarations, make sure the actual elements in the document match the rules set down in the DTD.