.app {
    text-align: center;
}

.big-cell {
    border: 1px black solid;
    display: inline-block;
}

.cell {
    border: 1px gray solid;
    width: 20pt;
    height: 100%;
    display: inline-block;
    text-align: center;
}

.cell-highlight {
    color: red;
}

span.cell:empty:before {
  content: "\200b"; /* unicode zero width space character */
}