When using elements like img, div and such you only want to apply styles that would be needed for ever one of those on your page. Generally speaking you would not use any kind of positioning on those generic elements.
You would use things like color and font size.

.left {
width: 30%;
border: 3px solid #000;

-moz-box-sizing: border-box;
-webkit-box-sizing: border-box;
-ms-box-sizing: border-box;
box-sizing: border-box;
}
Use the box-sizing: border-box property. It modifies the behaviour of the box model to treat padding and border as part of the total width of the element (not margins, however). This means that the set width or height of the element includes dimensions set for the padding and border. In your case, that would mean the element's width and it's border's width would consume 30% of the available space.