<div class="content">
    This is the content you want to mask.
    <div class="mask"></div>
</div>
.content {
    position: relative;
    z-index: 1;
}

.mask {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-color: black;
    opacity: 0.5;
    z-index: 2;
}

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External JavaScript

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