css 2 divs nebeneinander

1            #right{
2                 background: green;     
3                 width: 100%;
4               }
5               #left {
6                 margin-top: 5px; /* to test if they intersect*/
7                 background: red;
8               }  
9               #container {
10                  width: 800px;
11               }
12               <div id="container">
13                  <div id="left"> This div is as big as it's content</div>
14                  <div id="right"> rest of space</div>
15               </div>