Question about HTML abbr and acronym tags

What is the intent of the HTML standard on handling plurals of
acronyms and/or abbreviations?  Is it expected that user agents
(especially non-visual ones) will understand something such as

	<abbr>ISP</abbr>s

Or is it necessary to use

	<abbr>ISPs</abbr>

Thanks!

-Wayne Pollock, pollock@acm.org