Make children divs the height of tallest child

Answer 1

One solution is to change the display value from inline-block to table-cell for the descendant div.item elements:

.row {
    border: 1px solid red;
    display: table;
    border-spacing: 20px; /* For controlling spacing between cells... */
}
.item {
    background: rgba(0,0,0,0.1);
    display: table-cell;
}

Example