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.inner-border {
    background: #000;
    color: #fff;
    margin: 50px;
    padding: 15px;
    position: relative;
}

.inner-border:before {
    border: 5px solid #000;
    content: "";
    position: absolute;
    top: -10px;
    bottom: -10px;
    left: -10px;
    right: -10px;
}