This section is informative.
This Section defines the SMIL 2.1 Structure module. The Structure module provides the base elements for structuring SMIL content. These elements act as the root in the content model of all SMIL Host Language conformant language profiles. The Structure module is a mandatory module for SMIL Host Language conformant language profiles.
The SMIL Structure module is composed of the smil, head, and body elements, and is compatible with SMIL 1.0 [SMIL10]. The corresponding SMIL 1.0 elements form a subset of the Structure module, both in syntax and semantics, as their attributes and content model is also exposed by the Structure module. Thus, the Structure module is backwards compatible with SMIL 1.0.
The SMIL 2.1 specification leaves the SMIL 2.0 Structure Module unchanged.
Please refer to the former SMIL 2.0 Structure Module Module [SMIL20-structure] for information about semantics, elements and attributes for structuring SMIL content.