Result Size: 625 x 664
x
 
<!DOCTYPE html>
<html>
<head>
<style> 
.flex-container {
  display: -webkit-flex;
  display: flex;
  width: 400px;
  height: 250px;
  background-color: lightgrey;
}
.flex-item {
  background-color: cornflowerblue;
  width: 75px;
  height: 75px;
  margin: auto;
}
</style>
</head>
<body>
<div class="flex-container">
  <div class="flex-item">Perfect centering!</div>
</div>
</body>
</html>