Handling the before and after scenarios
Specify how an animation's styles are applied while awaiting start and after completion.
One of these keyword values:
|none||No special rules for before or after the animation|
|backwards||Before the animation starts, pre-apply the initial property values|
|forwards||When the animation is complete, keep the property values reached in the final state|
|both||Preapply initial property values, and keep final state afterwards|