/* optional */

li {
    line-height: 2;
}

:invalid {
    outline: 1px solid red;
}

.shell {
    position: relative;
    line-height: 1;
}

    .shell span {
        position: absolute;
        left: 3px;
        top: 1px;
        color: #ccc;
        pointer-events: none;
        z-index: -1;
    }

        .shell span i {
            font-style: normal;
            /* any of these 3 will work */
            color: transparent;
            opacity: 0;
            visibility: hidden;
        }

    input.masked,
    .shell span {
        font-size: 16px;
        font-family: monospace;
        padding-right: 10px;
        background-color: transparent;
        text-transform: uppercase;
    }