>>column-gap has a limited support: http://caniuse.com/#feat=multicolumn
>
>Ok
>
>>I would rather have a positive margin on images and a negative padding on the containing div (à la bootstrap row / col)
>
>Interesting trick, I would have to work a little bit with that to see if that would work.


Inspired from bootstrap (http://getbootstrap.com/css/#grid-example-basic) though I did it myself in the past.

The principle behind this trick is the CSS box model 'collapsing margins': https://www.w3.org/TR/CSS21/box.html#collapsing-margins