The cascade is an algorithm that defines how user agents combine property values originating from different sources. The cascade defines the origin and layer that takes precedence when declarations in more than one origin, cascade layer, or @scope block set a value for a property on an element.| developer.mozilla.org
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
The @keyframes CSS at-rule controls the intermediate steps in a CSS animation sequence by defining styles for keyframes (or waypoints) along the animation sequence. This gives more control over the intermediate steps of the animation sequence than transitions.| developer.mozilla.org
CSS has been on fire lately, with tons of great new features. @starting-style is an interesting one; it allows us to use CSS transitions for enter animations, something previously reserved for CSS keyframe animations. But is the juice worth the squeeze?| Josh Comeau's blog