First text only elements. This means elements that take no other elements as content.

<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT subject (#PCDATA)>
<!ELEMENT message (#PCDATA)>

Then let us look at empty elements. This means no content, they may have attributes. Think of XHTMLs <img ... />

<!ELEMENT br EMPTY>
<!ELEMENT hr EMPTY>
<!ELEMENT img EMPTY>

Finally the elements of mixed content, the block elements that may contain an array of other elements. We have so far seen the root elements that are defined by the DTDs but there are obviously others.

<!ELEMENT div (#PCDATA|b|u|i|em|p)*>