Some CSS variables are computed in JS, and the rest is made by CSS and the browser. Each item is a <div>:

<div class="wall" style="
  --start-x: 2560;
  --start-y: -2112;
  --end-x: 2560;
  --end-y: -2496;
  --floor-z: 32;
  --ceiling-z: 88;
">

All the geometry calculation is made in the browser.

JavaScript passes raw DOOM data in. CSS does the trigonometry.
JavaScript runs the game loop. CSS does the rendering.