<!doctype html>
<style>
  * {
    margin: 0;
  }
  .outer {
    position: absolute;
    overflow: hidden;
    border-radius: 9999px;
    width: 300px;
    height: 300px;
  }
  .inner {
    position: absolute;
    top: 0px;
    bottom: 0px;
    width: 300px;
    height: 300px;
    background: mediumspringgreen;
  }
</style><body><div class="outer"><div class="inner"></div>
