Concave corners

<div class="concave-corners">
<!-- The top-left corner -->
<div class="concave-corners__corner concave-corners__corner--tl"></div>
<!-- The top-right corner -->
<div class="concave-corners__corner concave-corners__corner--tr"></div>
<!-- The bottom-left corner -->
<div class="concave-corners__corner concave-corners__corner--bl"></div>
<!-- The bottom-right corner -->
<div class="concave-corners__corner concave-corners__corner--br"></div>
<!-- Content -->
...
</div>
:root {
--concave-corners-background: rgba(0, 0, 0, 0.3);
--concave-corners-size: 1rem;
}
.concave-corners {
background-color: var(--concave-corners-background);
/* Used to position the corners */
position: relative;
/* Misc */
height: 100%;
}
.concave-corners__corner {
/* Absolute position */
position: absolute;
/* Size */
height: var(--concave-corners-size);
width: var(--concave-corners-size);
background: #fff;
}
.concave-corners__corner--tl {
/* Position */
left: 0;
top: 0;
/* Border radius */
border-radius: 0 0 var(--concave-corners-size) 0;
}
.concave-corners__corner--tr {
/* Position */
right: 0;
top: 0;
/* Border radius */
border-radius: 0 0 0 var(--concave-corners-size);
}
.concave-corners__corner--bl {
/* Position */
bottom: 0;
left: 0;
/* Border radius */
border-radius: 0 var(--concave-corners-size) 0 0;
}
.concave-corners__corner--br {
/* Position */
bottom: 0;
right: 0;
/* Border radius */
border-radius: var(--concave-corners-size) 0 0 0;
}

See also

Inverted corners