-1
Just use the linear-gradient on the diagonal does not have much secret
body {
background-color: azure;
}
.box {
width: 100px;
height: 100px;
border: 7px solid #666;
border-radius: 50%;
background-image: linear-gradient(to top left, transparent 47%, red 48%, red 53%, transparent 54%)
}
<div class="box"></div>
Have you tried anything? What was the result?
– Woss