From Tutorialzine –Getting Started With The JavaScript Web Animation API:
Adding animations to web interfaces makes pages and apps feel more responsive and interactive. A side menu that smoothly slides out of view provides a much better user experience then a menu that just disappears when you close it. So far creating web animations was done either via CSS transitions, CSS keyframes, or an external library such as Animate.css or Velocity. Thanks to a new native JavaScript API, we are now able to freely animate any HTML element without ever having to leave our .js file.