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<!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>