Remove old -moz prefixed rules from viewer.css

This PR removes a number of old `-moz` prefixed CSS rules that have been supported in their non prefixed versions since Firefox 16.0, [released 2012-10-09](https://wiki.mozilla.org/RapidRelease/Calendar). The following prefixed rules are removed/changed:

* transition, https://developer.mozilla.org/en-US/docs/Web/CSS/transition.
 
* transition-property, https://developer.mozilla.org/en-US/docs/Web/CSS/transition-property.

* transition-duration, https://developer.mozilla.org/en-US/docs/Web/CSS/transition-duration.

* transition-timing-function, https://developer.mozilla.org/en-US/docs/Web/CSS/transition-timing-function.

* @keyframes, https://developer.mozilla.org/en-US/docs/Web/CSS/@keyframes.

* animation, https://developer.mozilla.org/en-US/docs/Web/CSS/animation.

For those worried about the file size of `viewer.css`, this should be a welcome change since these changes corresponds to a reduction of 1188 bytes (or approx. 3.5%).