Add Loader until stl image load PHP

<body style="margin: 0px">
    <div
      id="stl_cont"
      style="
        width: 600px;
        height: 600px;
        margin: 0;
        background: #ebebeb;
        overflow: hidden;
      "
    >
      <div
        class="loading"
        style="
          width: 600px;
          height: 600px;
          display: flex;
          align-items: center;
          justify-content: center;
        "
      >
        <img src="./assets/images/icons8-dots-loading.gif" alt="" />
      </div>
    </div>

    <script src="stl_viewer.min.js"></script>
    <script>
      var stl_viewer = new StlViewer(document.getElementById("stl_cont"),
        {
          models: [
            { filename: "/test/Sienna.stl" }
          ],
          all_loaded_callback: function () {
            document.querySelector(".loading").remove();
          },
        }
      );
    </script>
  </body>