Suppose i want a border-bottom to look like this:

This is not possible within default borders, as border-radius controls the radius around the element, not a single border edge.

In-order to achieve this, a pseudo element can be used.

div {
  max-width:50vw;
  padding-bottom:25px;
  position:relative;
}
div:after {
  content:'';
  position:absolute;
  bottom:0;
  left:0;
  right:0;
  background:red;
  height:20px;
  border-radius:0 10px 10px 0;
}

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The output would look like: