用 CSS 实现画三角形

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <div class="rectangle">

    </div>
  </body>

  <style>
    .rectangle {
      height: 0;
      width: 0;
      border-left: 50px solid transparent;
      border-right: 50px solid transparent;
      border-top: 0;
      border-bottom: 50px solid #f40355;
    }
  </style>
</html>