HTML:
<div class="container">
<div class="child"></div>
</div>
CSS:
.container {
height: 500px;
width: 500px;
display: flex; // Use Flexbox
align-items: center; // This centers children vertically in the parent.
justify-content: center; // This centers children horizontally.
background: white;
}
.child {
width: 100px;
height: 100px;
background: blue;
}