If you resize the browser to align with the screen blocks on the screen, you will realize that larger widths in your design can cause havoc to people using smaller resolution screens, which can be due to any number of reasons.
Another point you should note is that, even if the screen resolution is set to 800 x 600 the actual visible amount is much smaller than that in the browser.
For People using Screen Readers, this page consists of blocks of different screen resolutions with their respective screen resolutions following them, each smaller block is inside a larger block of different background color.
Back to the tutorial