[css] How to position the div popup dialog to the center of browser screen?

It took a while to find the right combination, but this seems to center the overlay or popup content, both horizontally and vertically, without prior knowledge of the content height:

HTML:

<div class="overlayShadow">
    <div class="overlayBand">
        <div class="overlayBox">
            Your content
        </div>
    </div>
</div>

CSS:

.overlayShadow {
    display: table;
    position: fixed;
    left: 0px;
    top: 0px;
    width: 100%;
    height: 100%;
    background-color: rgba(0, 0, 0, 0.75);
    z-index: 20;
}

.overlayBand {
    display: table-cell;
    vertical-align: middle;
}

.overlayBox {
    display: table;
    margin: 0 auto 0 auto;
    width: 600px; /* or whatever */
    background-color: white; /* or whatever */
}

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