What Does the SMIL begin Attribute Specify?

The begin attribute on a Synchronized Multimedia Integration Language (SMIL) timed media element specifies the exact moment when the element becomes active and starts its presentation or playback. By defining when media such as video, audio, or animation begins relative to a timeline, user interaction, or sibling elements, the begin attribute acts as a core mechanism for synchronizing complex, multi-track multimedia layouts across web standards and SMIL-based animation formats like SVG.

Core Function of the begin Attribute

In SMIL timing architectures, every timed element possesses an active timeline. The begin attribute explicitly controls the start of this timeline by accepting one or more timing values (separated by semicolons). When the designated trigger or timestamp evaluates to true, the player transitions the media element from an inactive state to an active playing state. If the attribute is omitted, the default value typically resolves to 0s, causing the element to play immediately when its parent synchronization container starts.

Supported Timing Value Types

The begin attribute supports a wide variety of timing specifications to handle static schedules as well as dynamic runtime events:

• Clock Values (Offsets): Explicit time representations specifying an offset relative to the document or parent container's start (for example, begin="5s" or begin="01:30").
• Syncbase Values: Relative timing based on the start or end of another timed element by referencing its ID (for example, begin="video1.end" or begin="intro.begin+2s").
• Event Values: Triggers fired by DOM or user-interaction events (for example, begin="button.click" or begin="mouseenter").
• Wallclock Values: Real-world date and time definitions using standard clock syntax (for example, begin="wallclock(2026-09-01T09:00:00Z)").
• Repeat Values: Triggers based on the iteration cycle of another looping element (for example, begin="anim1.repeat(2)").
• Indefinite: Defines that the element will not start automatically on a timeline and must be triggered externally via scripting methods like beginElement() (using begin="indefinite").

Interaction and List Processing

When multiple begin conditions are declared as a semicolon-separated list (for example, begin="click; prevMedia.end+5s"), the SMIL engine evaluates each condition independently. The element activates whenever any of the listed conditions are met, allowing for flexible playback controls that respond to both scheduled timelines and real-time user input.