A Better Animation System

Now the animation system (the animation extension) is heavily based on hard coded `animation-delay`.

I think maybe it can be more flexible (for example, animating multiple visible extensions) if we use the css counter as the delay value.

Start by locating the animation extension and its hard-coded animation-delay values. Read how visible extensions are selected and how the current delay is applied, then assess whether a CSS counter can support multiple visible extensions. Done means the animation behavior is more flexible without relying on fixed delays.

Written by the indexing model from the issue text.