Replace, add, or accumulate? That’s the question.
Combining multiple animation effects
The animation-composition property allows control of what should happen when multiple animations affect the same property simultaneously.
Say you have this base transform applied to an element:
transform-origin: 50% 50%;
transform: translateX(50px) rotate(45deg);
And that you also have this set of keyframes:
@keyframes adjust {
to {
transform: translateX(100px);
}
}
When applying these keyframes to an element, the transform in the to keyframe replaces the existing transform. This is the default behavior.
With animation-composition, you now have control over what should happen instead of the default replace. Accepted values are:
replace: The effect value replaces the underlying value. (default)add: The effect value is added to the underlying value.accumulate: The effect value is combined with the underlying value.
The difference between addition and accumulation is subtle. For example, take the two values blur(2) and blur(3). When added together would this produce blur(2) blur(3), but when accumulated this would produce blur(5).
You could compare this with a cup that is filled with tea. When pouring milk in it this would happen:
replace: The tea gets removed, and is replaced by the milk.add: The milk gets added to the cup, but it remains layered on top of the tea.accumulate: The milk is added to the tea and, because they are both fluids, they mix nicely.
Demo
In the demo below there are three gray boxes that have that base transform and animation applied to them.
Even though these boxes have the same animation they yield a different result because they have a different animation-composition set:
The first box is set to animation-composition: replace. This is the default behavior. In its end position, the original translateX(50px) rotate(45deg) value for transform is simply replaced by translateX(100px).
The second box is set to animation-composition: add. In its end position, the translateX(100px) is added to the original translateX(50px) rotate(45deg), resulting in translateX(50px) rotate(45deg) translateX(100px). This moves the box by 50px, then turns it 45deg, and then moves it 100px.
The third box is set to animation-composition: accumulate. In its end position, the translateX(100px) will be mathematically added to the translateX(50px) from the original transform, resulting in a transformation of translateX(150px) rotate(45deg).