<!DOCTYPE html>
<html>
<head>
<style>
div {
  /* Add two mask layers and apply border-radius to the bottom-most layer. */
  mask: linear-gradient(red, blue) border-box no-clip, 6%;
  border-style: solid;
  border-top-left-radius: 24%;
}
</style>
</head>
<div></div>
</html>
