RT Chris Coyier
Whack the corners off of whatever.
clip-path: polygon(0 5%, 5% 0, 95% 0, 100% 5%, 100% 95%, 95% 100%, 5% 100%, 0% 95%)
Or better, for those non-zero and non-100's, use a fixed value and calc() to pull it down off 100, so they stay at 45-degree angles.
https://codepen.io/chriscoyier/pen/yvQQxM

https://twitter.com/chriscoyier/status/1575207757929943040