Acronym Dictionary vs. Acronym Finder
The <acronym> tag should not be used, because it is not a valid HTML element in the HTML 5 specification (see "<acronym> is not supported in HTML 5. Use <abbr> instead." or "Authors are to use <abbr> for abbreviations." in HTML 5 differences from HTML 4).
Acronyms should be enclosed by the HTML<abbr> tag instead. They should also be disambiguated. The easiest way to do this is simply find the term for the meaning of the acronym in the acronym directory, then click on the "get HTML code" link for that term and copy-and-paste the code provided.
Other sites(1) mention that many developers improperly use the HTML <acronym> tag because the <abbr> tag does not work in IE. The code provided here does work, however. Try it - simply hover your mouse over the "HTML" acronym in the previous paragraph above.
1) For example, see "ToolTips for text on the Web" at http://sob.Apotheon.org/?p=352 or search for abbr vs. acronym tag in IE in Google.