I tried using “float: left” But somehow it looks like the the dark gray box is overlapping the content, when I want it to move to the left but stay aligned with the content across from it.
Basically floats needs to be cleared, though an extra wrapper element around the (blog)sections here (also floated) helps. Then better put those styles for floats within a media-query, to also make the layout work on smaller screens.
Note the overflow: auto for clearing on the .wrapper in my fork of your demo: