From David:

http://dev.w3.org/csswg/css-animations/ is currently not clear on
what CSS animations should do in non-interactive media.  (The
"Media" lines in the spec are almost certainly wrong.)

For example, when printing, what happens to CSS animations?  There
are two obvious choices:

  (1) ignore the animation properties and don't apply any animations

  (2) honor the animation properties and freeze the animations at
      time 0

I tend to think the correct answer is (1); this allows authors to
get reasonable fallback when the initial state of their animation is
offscreen or similarly useless, and it matches the fallback they
already (should) have for implementations that don't support CSS
animations.

(It's not what Gecko currently implements, but I'm thinking of
changing it.)