Currently the Web Animations API allows developers to create animations using the Element.animate() method, which returns an Animation object that is internally constructed. The full Animation constructor gives the developer more control over the created Animation, including specifying the timeline (currently useless, but in the future enables other types of timelines) and the exact KeyframeEffect object (see https://www.chromestatus.com/feature/4868532347076608)
Explainers: https://drafts.csswg.org/web-animations-1/#the-animation-interface