Yea. Well pointed out, Andrew. I always add that at the top and it really does save a lot of the time. It's predominantly due to that Firefox, IE, Opera, et cetera... all stupidly set their very own margin and padding for various elements, and thus resetting all the values allows you to build from the ground up.
The man who comes back through the Door in the Wall will never be quite the same as the man who went out.
If the sum of the divs widths is less than the full width of the space available they both float to the left and bump into each other, so to speak, and end up sitting next to each other. If you don't float them both, one drops below the other. Dropping below the other can also happen if you add borders and margins and padding and the sum of their total width exceeds the total space available. you can also get this if you use percentages as widths and rounding off errors take you over the 100% mark, so I often make the percentages add up to 95% instead.