HTML allows authors to associate any number of external style sheets with a
document. The style sheet language defines how multiple external style sheets
interact (for example, the CSS "cascade" rules).
Authors may specify a number of mutually exclusive style sheets called alternate
style sheets. Users may select their favorite among these depending on their
preferences. For instance, an author may specify one style sheet designed for small
screens and another for users with weak vision (e.g., large fonts). User agents
should allow users to select from alternate style sheets.
The author may specify that one of the alternates is a preferred style sheet. User
agents should apply the authorís preferred style sheet unless the user has selected
a different alternate.
Authors may group several alternate style sheets (including the authorís preferred
style sheets) under a single style name. When a user selects a named style, the
user agent must apply all style sheets with that name. User agents must not apply
alternate style sheets with a different style name. The section on specifying external
style sheets [p.191] explains how to name a group of style sheets.
Authors may also specify persistent style sheets that user agents must apply in
addition to any alternate style sheet.