This sentence uses white space to separate words.
Unless you tag every word, that white space has meaning. It is
not purely presentational.
Similarly with other uses of white space: if one represented by
tagging everything white space can be used for, you wouldn't
need it. But in a small tag set such as HTML, you can't get
along without it. Even in a large SGML DTD provision is not
made for all the things one might use white space for,
-- Terry Allen (email@example.com) Editor, Digital Media Group O'Reilly & Associates, Inc. Sebastopol, Calif., 95472