CSS Generators

.box {
  --mask:
    radial-gradient(40px at 0    0   ,#0000 98%,#000) 0    0   /51% 51%  no-repeat,
    radial-gradient(40px at 100% 0   ,#0000 98%,#000) 100% 0   /51% 51%  no-repeat,
    radial-gradient(40px at 100% 100%,#0000 98%,#000) 100% 100%/51% 51%  no-repeat,
    radial-gradient(40px at 0    100%,#0000 98%,#000) 0    100%/51% 51%  no-repeat;	
  -webkit-mask: var(--mask);
          mask: var(--mask);
}