Glossary

A syntactic construct of a document type definition (DTD) for its application. 

A piece of content with associated behaviors that can be activated through the user interface or through an API. The set of elements that a user agent enables is generally derived from, but is not limited to, the set of interactive elements defined by implemented markup languages

Text equivalents (e.g., text equivalents for images, text transcripts for audio tracks, or collated text transcripts for a movie) and non-text equivalents(e.g., a prerecorded audio description of a visual track of a movie, or a sign language video rendition of a written text).

User interface events, changes to content, loading of content, and requests from the operating environment.