Rationale
Abbreviations and acronyms can be confusing to anyone who does not know their correct meaning, and can be completely indecipherable when rendered by speech or braille output software.
When coded correctly using ABBR (abbreviation) and ACRONYM (acronym) text can be displayed in full via a "tooltip" (when a user mouses over text) or read in full by a screen reader.
Bear in mind however that "tooltips" can be a problem for some users. Firstly, not all users can use a mouse, for example somebody with upper body mobility problems, and therefore they will not be able to see the "tooltip". Secondly users with low vision may also not be able to read a "tooltip" as it cannot be resized via the browser. Finally, Internet Explorer currently does not support ABBR.
Techniques
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Expand the first occurrence of the abbreviation or acronym on the page. For example:
<ABBR title="General Accounting Office">GAO</ABBR>
<acronym title="Disk Operating System">DOS</acronym>
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Explain the first occurrence of an acronym or abbreviation after the first instance of it on each page. This can be done instead of using ABBR or ACRONYM. This could be the full text provided in brackets after the acronym or abbreviation.
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LANG attributes should be included if the acronym or abbreviation is in a different language.
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A glossary is a useful way of providing explanations of abbreviations and acronyms, especially if your site contains a lot of specialist information. Ideally this should be linked to form all pages in the site.
Testing tips
Verify if there are acronyms or abbreviations and look for expansions and explanations.
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Browser - Check for acronyms and abbreviations on pages manually. See if there is an explanation in brackets or a link to a glossary page.
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Screen reader - Acronyms and abbreviations will be read out as if the are a word, for example RNIB is read as "renib". Listen to pages for any odd sounding words.
Compliance with WCAG 1.0
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14.1 Use the clearest and simplest language appropriate for a site's content. Priority 1
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4.2 Specify the expansion of each abbreviation or acronym in a document where it first occurs. Priority 3
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14.3 Create a style of presentation that is consistent across pages. Priority 3
For more information on techniques visit the Web Accessibility Initiative techniques page.