How to create a heart shape using CSS?

Question

I\'ve found the following CSS in one of the answers here on SO and I was wondering why does it create the desired heart shape:

Answer 1

Here is another idea using one element and relying on multiple backgrounds to achieve the heart shape. You can also easily adjust the size by only changing the width:

.heart {
  width:200px;
  background:
   radial-gradient(circle at 60% 65%, red 64%, transparent 65%) top left,
   radial-gradient(circle at 40% 65%, red 64%, transparent 65%) top right,
   linear-gradient(to bottom left, red 43%,transparent 43%) bottom left ,
   linear-gradient(to bottom right,red 43%,transparent 43%) bottom right;
  background-size:50% 50%;
  background-repeat:no-repeat;
  display:inline-block;
}
.heart::before {
  content:"";
  display:block;
  padding-top:100%;
}

You can also use mask and you can have any kind of coloration:

.heart {
  width:200px;
  display:inline-block;
  -webkit-mask:
     radial-gradient(circle at 60% 65%, red 64%, transparent 65%) top left,
     radial-gradient(circle at 40% 65%, red 64%, transparent 65%) top right,
     linear-gradient(to bottom left, red 43%,transparent 43%) bottom left ,
     linear-gradient(to bottom right,red 43%,transparent 43%) bottom right;
  -webkit-mask-size:50% 50%;
  -webkit-mask-repeat:no-repeat;
  mask:
     radial-gradient(circle at 60% 65%, red 64%, transparent 65%) top left,
     radial-gradient(circle at 40% 65%, red 64%, transparent 65%) top right,
     linear-gradient(to bottom left, red 43%,transparent 43%) bottom left ,
     linear-gradient(to bottom right,red 43%,transparent 43%) bottom right;
  mask-size:50% 50%;
  mask-repeat:no-repeat;
  background:linear-gradient(red,blue);
}
.heart::before {
  content:"";
  display:block;
  padding-top:100%;
}

How does it works?

The whole shape is combined using 4 gradients: 2 gradients to create the top part and 2 for the bottom parts. each gradient is taking 1/4 of size and placed at a corner.

Use a different color for each gradient to clearly identify the puzzle

.heart {
  width:200px;
  background:
   radial-gradient(circle at 60% 65%, red  64%, grey 65%) top left,
   radial-gradient(circle at 40% 65%, blue 64%, black 65%) top right,
   linear-gradient(to bottom left, green 43%,black 43%) bottom left ,
   linear-gradient(to bottom right,purple 43%,grey 43%) bottom right;
  background-size:50% 50%;
  background-repeat:no-repeat;
  display:inline-block;
  border:5px solid yellow;
}
.heart::before {
  content:"";
  display:block;
  padding-top:100%;
}