<!DOCTYPE html>
<html>
<!--
 | Demonstrate visibility hidden:
 | The generated box is invisible (fully transparent, nothing is drawn).
 |   https://www.w3.org/TR/CSS21/visufx.html#propdef-visibility
 -->
<head>
  <style>
    body {
      font-family: Roboto;
      font-size: 20px;
      margin: 0px;
    }

    div {
      display: inline-block;
    }

    .large {
      width: 50px;
      height: 20px;
    }
    .small {
      width: 20px;
      height: 20px;
    }
    .red {
      background-color: #F00;
    }
    .green {
      background-color: #0F0;
    }
    .blue {
      color: #FFF;
      background-color: #00F;
    }
    .hidden {
      visibility: hidden;
    }
    .visible {
      visibility: visible;
    }
  </style>
</head>
<body>
  <div class='large green'>
    <div class='small red hidden'></div>
    <video class='small blue hidden'></video>
  </div>
</body>
</html>