After a recent WordPress upgrade, I noticed that the acronym button is no longer available in the Tiny MCE editor plugin I use.
I am a big fan of generating complete, semantic HTML, and always take the time to add the extras.
The <acronym> tag has been deprecated with the release of HTML5 in favour of the <abbr> tag.
<abbr title="Hyper Text Markup Language" lang="en">HTML</abbr>
will result in:
HTML
It is available in the WP Edit plugin.