The switch mechanism encourages this type
of development and experimentation, but at the same time provides Authors who wish to take advantage
of it the security
of knowing that their content will still display on any compliant Reading
System (i.e., it maintains the baseline
requirement that all XHTML Content Documents be valid if
none of the specialized markup is supported).
It is recommended that Reading
Systems support at least one
of the [H. 264] and [VP8] video codecs, but this is not a conformance
requirement — a Reading
System might support other video codecs, or
none at all.