'pronunciation' rel value
The 'pronunciation' value is proposed for the 'rel' attribute of the 'link' element of HTML. This value signifies that the referenced document provides pronunciation rules suitable for speech synthesis (otherwise known as "Text To Speech", or TTS). Pronunciation instructions describe how specific text tokens should be rendered in the aural dimension using synthetic voice, and are expressed based on a given phonetic alphabet.
The file referenced by the 'link' element contains a set of rules that associates strings of characters (e.g. words, or other kinds of tokens) with pronunciation instructions. For example, a rule is able to define whether the word "tomato" should be spoken with the british or north-american accent.
These pronunciation rules should be processed by voice-enabled user-agents (e.g. web browsers that support TTS rendering), but conversely, "pronunciation" link/rel files can be completely ignored by systems that do not have speech capabilities.
When several pronunciation lexicons are referenced within a single HTML document, and/or when a pronunciation rule is specified more than once for a given string of characters, the last occurrence of the rule takes precedence (in document order), in such a way that any previously-defined pronunciation rule gets overridden.
The 'pronunciation' value is used in the real world by the EPUB 3.0 standard (currently in draft state, due to be published in stable form in 2011).
The format recommended for use is W3C PLS (Pronunciation Lexicon Specification), with the MIME type "application/pls+xml" (and usually, the extension ".pls").