"Mark up abbreviations and acronyms with ABBR and ACRONYM and use "title" to indicate the expansion:
<p>Welcome to the <acronym title="World Wide Web">WWW</acronym>!</p>
This also applies to shortened phrases used as headings for table row or columns. If a heading is already abbreviated provide the expansion in ABBR. If a heading is long, you may wish to provide an abbreviation [...].
<p><abbr title="Social Security Number">SS#</abbr></p>
Copyright © 2001-2003 by Rainer Hillebrand and Thomas Wierlemann