flex example

HTML

<div class="container">
<div class="child"></div>
<div class="child"></div>
</div>

CSS

* {
  padding: 0;
  margin: 0;
}
html, body {
  height: 100%;
}
ul {
  height: 100%;
}
.container {
  background: silver;
  width: 100%;
  height: 600px;
  display: flex;
  flex-flow: column nowrap;
  align-items: center;
  justify-content: center;
}
.child {
  height: 150px;
  width: 150px;
  margin: 10px;
  background: blue;

}