You may want to move the overflow from the side-bar to the UL
The default margin on the body is causing some overflow, zero it out.
Remove the negative margin-left on .main-doc at the 400px break point.
Give the content blocks some left/right padding.
About the formatting:
Overall I would say it looks pretty good, there is a bit of an issue with the indentation of the HTML at the end, I can’t tell what the two last closing divs par up with, I think there may be one closing div too many. But it can be really hard to tell on CodePen.
As for the CSS again it’s pretty good. If I am going to nitpick I would like to see the spacing of the selectors and the starting curly brackets be the same in the media queries as outside (i.e. one space after the opening curly bracket).