How can I make my border-radius curve outward?

Question

I am try to make a div have borders like the following drawing:

This is what I have tried:

div {
    height: 100px;
    width: 100px;
    border-bottom-right-radius:100px 10px;
    border:1px solid #000;
}

<div></div>

What is an effect way to accomplish this?

Answer 1

Using :before and :after

The top border is created with the :before:

• Its height is the same as the border radius

• It is positioned just outside with top and lines up with the left border thanks to left

• Its width is calculated with calc to precisely line up the top of the curve

• The curve can be refined with transform: skewX(-60deg)

The left border is created with the :after:

• It is given a 100% height minus the height of the before and the thickness of the border with calc

Examples

Number 1 - a bit pointy

div {
  border-bottom-right-radius: 100px 20px;
  border: 1px solid #000;
  border-top: none;
  height: 500px;
  width: 200px;
  position: relative;
  border-left: none;
}
div:before,
div:after {
  content: '';
  display: block;
  position: absolute;
  left: -1px;
}
div:before {
  height: 20px;
  width: 100%;
  width: calc(100% + 1px);
  border-bottom-right-radius: 100px 20px;
  border-bottom: 1px solid #000;
  border-right: solid 1px #000;
  top: -1px;
}
div:after {
  height: calc(100% - 18px);
  border-left: 1px solid #000;
  top: 19px;
}

<div></div>

Number 2 - smoothed out point with skew

div {
  border-bottom-right-radius: 100px 20px;
  border: 1px solid #000;
  border-top: none;
  height: 200px;
  width: 200px;
  position: relative;
  border-left: none;
}
div:before,
div:after {
  content: '';
  display: block;
  position: absolute;
  left: -1px;
}
div:before {
  height: 20px;
  width: 100%;
  width: calc(100% - 36px);
  border-bottom-right-radius: 100px 20px;
  border-bottom: 1px solid #000;
  border-right: solid 2px #000;
  top: 0px;
  left: 17px;
  transform: skewX(-60deg);
}
div:after {
  height: calc(100% - 19px);
  border-left: 1px solid #000;
  top: 20px;
}

<div></div>