By adding position: absolute; it will still not be in the center as you have left: 50%; so the most left side of the start will start in the center the actual site wont be centered. You could possibly think well if its a screen width of say 800px without the scroll bar, the site is 500px Therefore 62.5% of the screen is taken up by the site, then there should be 18.75% on the left. However this will only be centered in one resolution any way so its a waste of time.

I agree with the others to use a container to to center the div tag however I am guessing if your heading is centered the rest of the site will be as well. Therefore there is no need for another div as a container, the body tag can be the container and you can set the css property text-align: center; to the body tag and then the heading and all other tags for your content, footer and whatever else will be centered.