> Why isn't the usual Markdown (or the like) approach enough?

Because you can't use markdown to design CSS or SVG animations. I want to be able to design an animation with code that describes how the elements move/interact and then compile that down into CSS or SVG keyframes so that I don't have to manually declare however many arbitrarily complex keyframes.

I understand the purpose of a static site generator and I'm not looking for anything to replace that. Rather I want to be able to write the templates for my static sites without 1. writing exceptionally tedious HTML & CSS and without 2. relying on client side JS to do those tedious things.