This div is set to 75% of the page width and 100px high.
This div is set to 500 pixels in width and 100px high. If you shrink the width of this page you'll see the difference in using pixels vs percentages.
This div is set to 50% of the page width and an auto height.
If you hover over this line it will make a list visible. When that happens you will see what setting height to auto does.