I’d be interested in a practical definition of logic truly separated from design in the context of web tech.

The two are so ridiculously intertwined. It’s already a problem within CSS, where layout systems and styling are mixed. Flexbox and grid have nothing to do with colors and gradients, which are again themselves orthogonal to positional properties. Fonts, paddings and margins (in 15 different units, look it up) add to the insanity. Now sprinkle in animations, box models and borders.