Right, but I find these demos interesting nonetheless - in that they reveal capabilities of HTML + CSS that, as a casual frontend developer, you wouldn't normally think of them as having.
It's not an exercise in efficiency, or even attempting to prove that HTML + CSS is somehow better than a JS version. I think it's about getting as much as you can out of CSS and HTML. As a viewer, looking at these projects usually give me 2-3 tricks to add to my collection and is worth the read.
I honestly don't think I'll ever NEED to implement a CSS+HTML calculator sans JS, but it's nice to know that it's possible and it could be something I look at if I want to implement a piece of it for something similar.