How to vertically center a div for all browsers?

body, html {
  background: #F5F5F5;
  box-sizing: border-box;
  height: 100%;
  margin: 0;
}

#center_container {
  align-items: center;
  display: flex;
  min-height: 100%;
}

#center {
  background: white;
  margin: 0 auto;
  padding: 10px;
  text-align: center;
  width: 200px;
}

<div id='center_container'>
  <div id='center'>I am center.</div>
</div>