Make it possible for user agents to determine when specific content such as a word or phrase is to be emphasized.
In written documents, emphasis can be created through grammar, syntax, and writing style. Emphasis can also be indicated through markup and by setting visual and/or auditory properties in an associated style sheet.
Example 1. A written text that creates emphasis through grammar and writing style. "I. Told. You. Not. To. Do. That," he said through clenched teeth. (In this example, the deliberate use of one-word sentence fragments creates an impression that the speaker is very angry, and is pausing to emphasize each word. Further visual styling may be unnecessary.)
Example 2. A written text in which emphasis is indicated programmatically through markup and visually through typography. "I told you not to do that," he said through clenched teeth. (The use of italics in this example indicates that the word "told" is to be spoken more forcefully than the other words in the sentence. The sentence contains semantic markup, and the style sheet specifies that visual media should render the word in italics while speech output devices should slightly increase pitch and speech rate.)
Example 3. An auditory-only presentation delivered over the telephone. Emphasis is created entirely through phonetic and lexical markup. (SSML Techniques, other data models for controlling prosody.)