In order to avoid this unexpected fail, we add this tentative
serialization for animation shorthand.
1. animation-timeline is reset-only in this shorthand, so we don't parse
it and we reset it to `vec![auto]`.
2. we don't serialize the shorthand if animation-timeline is not the initial
value.
This is a tentative solution and we should update it with the new syntax
once the spec gets update (https://github.com/w3c/csswg-drafts/issues/6946).
Differential Revision: https://phabricator.services.mozilla.com/D217164
1be62642df