html5 help please

Hey I need some help. Basically the width of the main body ( section ) after the nav, not place itself under the nav taking up 100% width. It only does so if I put in a specified number of pixels. Basically a one column site page.

Here is the code:

#section {
width:100%;
height:150px;
background-color:#ccc;
margin-right:auto;
margin-left:auto;
}

 

I'm having trouble understanding the problem and what you said, can you reiterate?

note) isn't this an id in CSS?