CSS is very extremely well-specified in how it draws stuff, and if you use it within its bounds, it's very good at doing what it set out to accomplish. That is, as a way to style content. It was not designed as a "draw a triangle" language, and I'm sure the original people behind the CSS standard would never have thought of exploiting the bevel effect to draw a triangle.

I don't understand the attitude of "well, you're using CSS outside the bounds of what it was designed for, that means that CSS is ad-hoc garbage", nor the attitude of "well, you're exploiting CSS in a clever way, that means you're a terrible engineer".

It's a clever hack in the truest sense of the word, and that makes me proud.