border-radius-split-background.html

<style>
  div {
    display: inline-block;
    border: 1px solid black;
  }
  span {
    font-size: 100px;
    background-color: gray;
    line-height: 1.5em;
    border: 2px solid black;
    border-radius: 50%;
  }
  
  .vertical {
    -webkit-writing-mode: vertical-rl;
    -webkit-text-orientation: sideways-right;
  }
</style>
<body>
  <div class="horizontal">
    <span>&nbsp;&nbsp;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span>
  </div>
  <div class="vertical">
    <span>&nbsp;&nbsp;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span>
  </div>
</div>

chromium/third_party/blink/web_tests/fast/backgrounds/border-radius-split-background.html