The file has no namespace or DOCTYPE declaration, the user agent should attempt
to play this as a SMIL 1.0 document.
Strict SMIL 1.0 implementations will issue an error on the unknown attribute
value. Permissive SMIL 1.0 implementations may ignore the unknown attribute
value, resulting in the end attribute being ignored, and the image and the
document having 0 duration, instead of the unresolved duration that it would
have in SMIL 2.1.