An element is said to have "element content" if its declaration matches children. The description of what this means only makes sense if you know enough computer science to be comfy with the concept of a "language generated by a regular expression." In English, it means that if, in the DTD, you've given rules for an element saying what other elements that can appear inside it and what order they have to appear in, then the document follows those rules.
There is an important point about white space. If I have a declaration saying, for example that a OL element is allowed to contain only LI elements, then it's OK for there to be white space between them; this makes it possible, among other things, for the LI elements to appear on separate lines.
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