Siege84

Add a bit of overflow to your CSS, eg:

nav { background: #555555; overflow:hidden; }
 
Long story short, when every element inside a container is set to "float", the container effectively has a height of 0px. So all the elements in that container will overlap the content below it (the <section> in your case). Adding an overflow value will tell the container to wrap around the floated elements and give it the appropriate height value.