video timing
video timing is tested without the dur attribute explicitely set, this
means the simple duration becomes the implicite media length, this
is the same as to set dur to 'media'.
The media consists of two frames, blue and yellow, each of
them for one second. A finite repetition is defined in the
GIF, therefore the media length is the repeated length of 8s.
(according to the original browser Netscape 4 of the inventor of the
looping extension).
If GIF is not supported, the media duration cannot be derived
from the source, therefore it becomes indefinite.
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-magenta rotating square behind with the same begin and end using
simple offset values.
switch: GIF ok
switch: no GIF