Don't add composite or neutral as CSS properties of KeyframeInternal
Don't add composite or neutral as CSS properties of KeyframeInternal

Also add a couple of asserts for compositing.

web-animations/web-animations-js-legacy,birtles/web-animations-js,web-animations/web-animations-js-legacy,birtles/web-animations-js,web-animations/web-animations-js-legacy,birtles/web-animations-js,beni55/web-animations-js-legacy,beni55/web-animations-js-legacy,beni55/web-animations-js-legacy,beni55/web-animations-js-legacy