It is an all too easy trap to fall into. You make a page, and it looks great on your screen. Your cool background image lines up with your text, creating a really spiffy effect. You tell all your friends about it, wanting to show off your superior HTML coding skills. And they laugh at you.
Nothing lines up.
You have white text over the light yellow parts of the background image. You background image doesn't tile like you thought it did. Your tables are all squashed. More than likely you lulled yourself into thinking that everyone out there has the exact same size monitor as you do and runs the exact same resolution. Odds are, they don't.