SUMMARY: Use markup that facilitates pronunciation or interpretation of abbreviated or foreign text.
This standard briefly discusses the importance of making content available in the natural language. It also mentions the benefits of providing expansions of any abbreviations or acronyms used in the text.
By assuming the only people who will view your site are those who speak the same language as you do, you prevent a significant number of people who need that specific mark up for them to be able to access your stuff.
Developers can specify the dominant natural language of the site by using the "lang" attribute in HTML and use "xml:lang" in XML. Server administrators can configure their servers to take advantage of this feature and provide better search results based on natural language preferences.
This guideline also suggests, "[using] the "title" attribute of the ABBR and ACRONYM elements. Providing the expansion in the main body of the document also helps document usability." Some websites would just assume visitors will know what HTML or CSS, for example, means. This can discourage new users and also those who may not be familiar with acronyms for things they already know.