Micro clearfix. If you don’t know about this technique, you should. This technique shows you how to put a clearfix on a specific element so you don’t have to create an extra element for it. http://nicolasgallagher.com/micro-clearfix-hack/
You should also look at the box model code. This lets you not worry about the total math (content, padding, border) for your display: block elements, such as divs, sections, articles, etc. Paul Irish Flexbox
These things should probably be at the top of whatever CSS styles you use. Along with a copy of Normalize.css.