.input-group {
    position: relative;
    display: inline-block;
}

#myTextArea {
    color: black;
}

#myTextArea::after {
    content: "";
    display: none;
    position: absolute;
    left: 0;
    right: 0;
    bottom: -2px;
    height: 2px;
    background-color: red;
}

#myTextArea[data-contains-slash]::after {
    display: block;
}