<div class="container">
    <div class="panel"></div>
</div>
.container {
    width: 50%;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

.panel {
    background-color: red;
    width: 400px;
    height: 300px;
    margin: 0 auto;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.