There are two different ways to define the meaning of an acronym within an HTML document, the ACRONYM tag and the ABBR tag. In practise they are very similar, although their compatibility varies with browsers.
<acronym title="Hypertext Markup Language">HTML</acronym>
<abbr title="Extensible Markup Language">XML</abbr>
<ABBR> is only compatible with Netscape 6 upwards (Mozilla) whereas <ACRONYM> is supported by Internet Explorer 4 upwards and Netscape 6 upwards. It's probably a better choice to opt with <ACRONYM> in light of this.