<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
  </head>
  <body>
    <div class="triangle"></div>

    <script>
      var triangle = document.querySelector(".triangle");
      // 补全代码
      let s = "";
      for (let i = 0; i < 3; i++) {
        s += `${"*".repeat(i + 1)}</br>`;
      }
      triangle.innerHTML = s;
    </script>
  </body>
</html>