NOTE This is similar to this post, however there is a strict no-javascript requirement and the answer should be responsive (i.e there is not a fixed number of columns).
I would like to style several fixed width, variable height boxes using no javascript such that they form a kind of fluid grid. This jsfiddle yields the picture below. This is essentially just the following css:
div {
float: left;
margin: 1em;
border: 1px solid #999;
width: 150px;
}

I'd like for the only vertical empty space to be the margins. In terms of the screenshot above, I'd like for the tops of 5 and 6 to move upwards to 1em from the bottom of 1 and 2, respectively.
float:leftis not going to allow for your desired result without the ability to calculate the block height and respond to it. ie. jQuery/jsflexbox(AFAIK) doesn't offer any help here. That's why the JS solutions are so popular.