XML in a Nutshell, 3rd Edition

Name

Proper Declaration/PE Nesting

Text from specification

Parameter entity replacement text must be properly nested with markup declarations. That is to say, if either the first character or the last character of a markup declaration is contained in the replacement text for a parameter entity reference, both must be contained in the same replacement text.

Explanation

This constraint means you can’t create a parameter entity that completes one DTD declaration and begins another; the following XML fragment would violate this constraint:

<!ENTITY % finish_it ">">
<!ENTITY % bad "won't work" %finish_it; <!--wrong!-->