如何用HTML5 实现下面问题呢

用盒子模型知识做三角形和圆形图，是利用HTML5中盒子模型的知识在边框右下角做出三角形和圆形图片

<!DOCTYPE html>
<html>
  <head>
    <title>Circle and Triangle with Box Model</title>
    <style>
      .shape {
        position: relative;
        width: 200px;
        height: 200px;
        border: 2px solid black;
      }
      .shape:before {
        content: "";
        display: block;
        border-radius: 50%;
        width: 100px;
        height: 100px;
        background: blue;
        position: absolute;
        top: 50%;
        left: 50%;
        transform: translate(-50%, -50%);
      }
      .shape:after {
        content: "";
        display: block;
        border: 20px solid transparent;
        border-bottom-color: red;
        position: absolute;
        bottom: -20px;
        right: -20px;
      }
    </style>
  </head>
  <body>
    <div class="shape"></div>
  </body>
</html>

img