Finally I found a way to make borders around my floated divs collapse like on table cells. The key was to use box-shadow instead of border. So this is actually a trick to simulate borders.

I couldn't use display:table-cell because table cells don't get rearranged vertically the way I want in responsive scenarios.

You can try to resize the browser window to further explore how the "borders" behave.

The first container:

Hello, this is the first floated div

And this is the second

And finally the third one

The second container:

Hello, this is the first floated div

And this is the second

And finally the third one