composite

The composite property of a KeyframeEffect resolves how an element’s animation impacts its underlying property values.

To understand these values, take the example of a keyframeEffect value of blur(2) working on an underlying property value of blur(3).

• replace - The keyframeEffect overrides the underlying value it is combined with: blur(2) replaces blur(3).
• add - The keyframeEffect is added to the underlying value with which it is combined (aka additive): blur(2) blur(3).
• accumulate - The keyframeEffect is accumulated on to the underlying value: blur(5).

Read more on MDN

I recommend reading web.dev’s article on web-animations.