.cssgrid {
 display: grid;
 grid-template-columns: repeat(4, 1fr);
 grid-template-rows: auto;
 grid-auto-flow: row dense;
}
.cssgrid .wide { grid-column: auto / span 2; }
.cssgrid .tall { grid-row: auto / span 2; }
.cssgrid .tall .thumb { min-height: 400px; }
.cssgrid .big { grid-column: auto / span 2; grid-row: auto / span 2; }