Resize your browser window and watch how it affects the behaviour of each table. Notice the only table that does not expand or collapse to accommodate the window dimensions is the one that uses absolute cell widths and an invisible GIF in each cell.
Table with no cell widths
|No width specified||No width specified|
Table with relative cell widths
Table with absolute cell widths
Table with absolute cell widths and single-pixel GIF
Width = 100
Width = 250