For a trivial example like fading out, you're right, but if you want to make dynamically generated animations, hyperscript's features are very helpful (The Web Animations API is a boon, but browser support isn't there yet).

Separation of HTML/CSS/JS doesn't always mean separation of concerns. Every application has a different definition of a "concern", and if it doesn't align with the separation between languages, you don't get decoupled code, you intensify the symptoms of tight coupling.