How to round out corners when using CSS clip-path

Question

I want to be able to round out the 3 leftmost corners on this shape that I have created, any idea how that can be done?

Answer 1

You could use a child element and do a nested clip-path on that and the child's pseudo element. The parent will do a polygon clip on the shape first, then the pseudo will have an ellipse to round the borders. The clips will have a combined effect.

.parent, .parent div, .parent div:before {
  width: 423px;
  height: 90px;
  position: absolute;
}

.parent {
  right: 0;
  background-image: linear-gradient(to right, transparent 210px, #b0102d 210px);
  margin-top: 15vh;
}

.parent div {
  clip-path: polygon(100% 0%, 100% 100%, 25% 100%, 0 50%, 25% 0);
}

.parent div:before {
  content: "";
  background-color: #b0102d;
  clip-path: ellipse(200px 45px at 210px);
}

Here is the demo with some adaptations to illustrate what's going on:

.parent, .parent div, .parent div:before {
  width: 423px;
  height: 90px;
  position: absolute;
}

.parent {
  right: 0;
  background-image: linear-gradient(to right, transparent 210px, yellow 210px);
  margin-top: 15vh;
}

.parent div {
  background-color: blue;
  clip-path: polygon(90% 0%, 90% 100%, 25% 100%, 0 50%, 25% 0);
}

.parent div:before {
  content: "";
  background-color: #b0102d;
  clip-path: ellipse(200px 45px at 210px);
}

The horizontal size and position of the ellipse can be used to get a different effect on the edges. Note that the background starting postion of the parent needs to be adjusted to the same value as the placement of the ellipse (last value in the clip-path) because it fills up whatever gets clipped off on the right side. This can be visualised by removing background-color: blue from .parent div in the second demo.

Here is an additional Codepen to to try it out.