Having thoroughly beat up the abbr element, it’s time to turn our attention to its close sibling (but definitely not twin) acronym. First, a quick note about the differences between abbreviations and acronyms. Yes, I know. We learned this in elementary school. But even the most brilliant of us sometimes forget — it’s a busy world out there, and we can’t always remember every little detail of every little thing. So:
Abbreviation: Making a word shorter by removing lots of letters. Some examples: etc, req’d, cont’d, or ms.
Acronym: Where a series of letters (usually the first letter of each word, which is why we get some pretty strangely named laws in the United States) takes the place of a word. Examples include: NBC, USPS, HTML.
In this day and age, you’re more likely to encounter an acronym than a million bucks, and figuring them out can be a full-time occupation. Using acronym is the best type of accessibility — it opens up the crazy world of acronyms to everyone. No longer will people be reading your website and scratching their heads over the crazy alphabet of non-words on your site.
Adding acronym to your documents is fun and easy. The basic code is:
<acronym title="United States Postal Service">USPS</acronym>
I know, I know — so little work for so much good.
Okay, here’s where things get a little tricky. It is considered good writing to spell out the meaning of an acronym the first time it is encountered in a document. Thereafter, it is presumed the reader can a) remember the meaning, or b) return to the original definition if their brains can’t retain the information. It is my belief that you only need to spell out abbreviations and acronyms on first usage — it is your call as to whether you wish to code each instance. As far as I can tell, the guidelines are flexible on this point (please correct me if I’m wrong), and I think most readers are accustomed to the long-established usage rules. That being said, it won’t hurt anything if you decide to go overboard.
Update: It’s possible the acronym element may become deprecated, the theory being that acronyms are a subset of abbreviations. Hmm. I guess I can buy that. Of course, standardizing on abbr won’t help when it come to Internet Explorer, because, unless a miracle happens and everyone upgrades to a new, magically compliant browser, it still won’t read abbr.
Update 2: The current draft for Web Content Accessibility Guidelines 2.0 offers specific advice about the use of abbr and acronym: first appearance. This conforms with standard writing rules, so should make things standard for everyone.
None of your examples are acronyms. NBC, USPS, and HTML are initialisms, not acronyms. Acronyms are pronounced as words, such as NAFTA and WCAG (when pronounced). All acronyms and initialisms are abbreviations, but not all abbreviations are acronyms.