opps, I forgot to include the link:
codepen.io/cbolson/full...

Returning to your original question, aren't seconds or milliseconds the only valid units for keyframe animations?

They are valid units so calc() doesn't have an issue with them.
So, as they are the same unit, calc() is cancelling them out to give 0.5, just as calc(5px / 10px) will also return 0.5

On the other hand calc(5px / 10hz) will fail as the units are not compatible (at least it does in my tests)

Are the CSS frequency units Hz and kHz still supported?
MDN suggests not.

I have now added more sliders to be able to control the second border-radius property. It can now generate even more interesting shapes!

I'd love one if you can ship to 🇪🇸 😅

I'm not sure happened to the preview image on that one so here is a short video capture