viewport

  object in which the user agent presents content

  The user agent presents content through one or more viewports. Viewports include windows, frames, loudspeakers, and virtual magnifying glasses. A viewport may contain another viewport (e.g., nested frames). Interface components created by the user agent such as prompts, menus, and alerts are not viewports.

  This definition is based on User Agent Accessibility Guidelines 1.0 Glossary [[UAAG10]].