What Is the Default Value of SMIL Restart?

This article examines the restart attribute in the Synchronized Multimedia Integration Language (SMIL), identifying its default value across major SMIL specifications and explaining how this setting governs multimedia and animation playback behavior.

Overview of the Restart Attribute in SMIL

In the Synchronized Multimedia Integration Language (SMIL) and its integrated profiles (such as SVG animation), the restart attribute dictates how timed elements react when triggered to begin while already running or after finishing playback. It helps developers control user interactions, event triggers, and synchronization rules to prevent unexpected overlapping or restarting of media clips and animations.

The Default Value Across SMIL Versions

Across all versions of SMIL that define the attribute—including SMIL 2.0, SMIL 2.1, and SMIL 3.0 (as well as the SVG animation specifications derived from SMIL)—the default value of the restart attribute is always.

SMIL 1.0

SMIL 1.0 did not include the restart attribute. Timing in SMIL 1.0 relied on a simpler model with static begin times, durations, and basic repeat attributes without granular restart controls.

SMIL 2.0 and SMIL 2.1

The restart attribute was formally introduced in SMIL 2.0 as part of the Timing and Synchronization module to handle advanced interactive scenarios and DOM event activations. The default behavior was established as restart="always".

SMIL 3.0

SMIL 3.0 retained the established timing model. The default value remained restart="always", ensuring complete backward compatibility with SMIL 2.x documents.

Valid Values and Behavior

The restart attribute accepts three discrete keyword values:

• always (Default): The element can be restarted at any point in time. If a begin condition or event occurs while the element is currently active, the current playback interval is terminated, and a new interval begins immediately.
• whenNotActive: The element can only be restarted if it is not currently playing. Any begin triggers or events that occur during the element's active duration are ignored.
• never: The element can only begin once per document lifecycle. Any subsequent attempts to trigger or re-activate the element are disregarded.

Controlling Defaults via restartDefault

SMIL specifications also provide the restartDefault attribute, which can be applied to parent time containers (such as <par>, <seq>, or <excl>) or document root elements.

When individual child elements do not explicitly declare a restart attribute, they inherit the behavior specified by the ancestor's restartDefault attribute. If no ancestor defines restartDefault, the standard fallback throughout the hierarchy remains always.