>It's a visual style, yet all your criticisms are about under-the-hood technicalities

the very point of brutalism is to turn its material 'inside out', what defines brutalism is that it makes the material and structural elements it's made of visible. It's not a 'visual style' or an aesthetic at all but an 'ethic of construction' something was brutalist if it was honest, and exposed, what a thing was made of.

If you want to apply that notion to web design It's not about 1 CSS file or 100 but whether the mechanics of CSS or HTML are still visible on the site, or if it's all buried under colors, animations, styling etc. Craigslist is a pretty good example of a brutalist website, so is this site, HN.