The @starting-style CSS at-rule is used to define starting values for properties set on an element that you want to transition from when the element receives its first style update, i.e., when an element is first displayed on a previously loaded page.| developer.mozilla.org
Learn about new capabilities that enable smooth transitioning of discrete animations and between the top layer| Chrome for Developers
1. Introduction| www.w3.org
New features in the Web Animations API allow us to interact with CSS Animations and Transitions.| danielcwilson.com