.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);
}