html,
body {
  width: 100%;
  height: 100%;
  padding: 0;
  margin: 0;
}

#prompt-form {
  display: inline-block;
  padding: 5px 5px 5px 70px;
  width: 200px;
  border: 1px solid black;
  background: white url(https://en.js.cx/clipart/prompt.png) no-repeat left 5px;
  vertical-align: middle;
}

#prompt-form-container {
  position: fixed;
  top: 0;
  left: 0;
  z-index: 9999;
  width: 100%;
  height: 100%;
  text-align: center;
}

#prompt-form-container:before {
  display: inline-block;
  height: 100%;
  content: '';
  vertical-align: middle;
}

#prompt-form input[name="text"] {
  display: block;
  margin: 5px;
  width: 180px;
}