video timing
video timing is tested with initialVisibility whenStarted and fill remove.
The media duration is 2s, one frame yellow for 1s and
one frame blue for 1s, the dur attribute is 4s with a begin at 2s.
This means the first yellow frame is displayed from the begin of
the active duration at 2s to 3s. The second blue frame is displayed from 3s
to 6s and is removed then. The initialVisibility is only used before
the first active duration, therefore it is not applied to the restart
at 8s, then the yellow frame is only displayed for 1s.
Because only the timing is tested, it is not important, whether
the used format animated GIF is interpreted or not; whether there is
some video content visible or not. This is tested bottom right with
a switch. Either in this area the animated GIF is displayed or
there is noted 'switch: no GIF' in black on white.
If 'switch: GIF ok' in black on red-magenta, the GIF is not displayed
and the switch for requiredFormats is wrong.
The timing is derived using the begin and end of the video as syncbases
for the begin and end for a rotation of a blue square.
This rotation reference is compared with a
red rotating square behind with the same begin and end using
simple offset values. If something red gets visible, an error is
occured.
switch: GIF ok
switch: no GIF