.separator {
    display: -webkit-box;
    display: -ms-flexbox;
    display: flex;
    -webkit-box-align: center;
        -ms-flex-align: center;
            align-items: center;
    text-align: center;
}

.separator::before,
.separator::after {
    content: '';
    -webkit-box-flex: 1;
        -ms-flex: 1;
            flex: 1;
    border-bottom: 1px solid #000;
    opacity: .25;
}

.separator:not(:empty)::before {
    margin-right: .25em;
}

.separator:not(:empty)::after {
    margin-left: .25em;
}