index-collapsible.html

<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <title>Test Site</title>
  <meta name="description" content="An interactive getting started guide for Brackets.">
  <style>
    .collapsible {
      background-color: #777;
      color: white;
      cursor: pointer;
      padding: 18px;
      width: 100%;
      border: none;
      text-align: left;
      outline: none;
      font-size: 15px;
    }

    .active,
    .collapsible:hover {
      background-color: #555;
    }

    .content {
      padding: 0 18px;
      display: none;
      overflow: hidden;
      background-color: #f1f1f1;
    }
  </style>
</head>

<body>
  <button type="button" class="collapsible">Click here</button>
  <div class="content">
    <p>Apparently you put the script in the body below all the collapsed content on the page</p>
  </div>
  <script>
    var coll = document.getElementsByClassName("collapsible");
    var i;

    for (i = 0; i < coll.length; i++) {
      coll[i].addEventListener("click", function () {
        this.classList.toggle("active");
        var content = this.nextElementSibling;
        if (content.style.display === "block") {
          content.style.display = "none";
        } else {
          content.style.display = "block";
        }
      });
    }
  </script>
</body>

</html>