Question

I’d like to get the following layout:

[ ][ ][ ][ ][ ][ ]
[ ][ ][ ][ ][ ][ ]
[ ][ ][ ][ ][ ][ ]
[ ][ ][ ][ ][ ][ ]

where each [ ] should be at most 300x150px, but scale down to the
bounding box as needed, conserving aspect ratio.

.field {
    width: 16%;
}

.field .placeholder{
    float: left;
    width: 300px;
    height: 150px;
}

I tried adding a placeholder div inside the divs, but that didn’t
really help, it gives me something akin to

[ ]
[ ]
[ ]
...

Answer 1

It looks like a table is what I need.

td { width: 1%; }

takes care of equal horizontally, but I still have the problem of vertical spacing.