DDICK / Firefox-Marionette-1.70 / t / data / visible.html 
<!DOCTYPE html>
<html lang="en-au">
  <head>
    <meta charset="utf-8">
    <title>Percentage Visible test</title>
    <style>
      .navbar {
        position: fixed;
        background-color: green;
        top: 0px;
        left: 0px;
        height: 25%;
        width: 100%;
        z-index: 10;
      }
      .content {
        background-color: yellow;
        height: 650px;
      }
      .display_none {
        position: fixed;
        background-color: blue;
        top: 0px;
        left: 0px;
        height: 100%;
        width: 100%;
        z-index: 20;
        display: none;
      }
      .visibility_hidden {
        position: fixed;
        background-color: red;
        top: 0px;
        left: 0px;
        height: 100%;
        width: 100%;
        z-index: 20;
        visibility: hidden;
      }
      .visibility_collapse {
        position: fixed;
        background-color: purple;
        top: 0px;
        left: 0px;
        height: 100%;
        width: 100%;
        z-index: 20;
        visibility: collapse;
      }
    </style>
  </head>
  <body>
    <div class="navbar">Navigation Bar</div>
    <div class="display_none">Should not see me</div>
    <div class="visibility_hidden">Should not see me either</div>
    <div class="visibility_collapse">Should have collapsed</div>
    <div class="content">Content</div>
    <form action="http://localhost">
      <input type="text" id="username"/>
    </form>
    <div class="content">Content</div>
  </body>
</html>