CSS Layout Backgrounds, of all the CSS visual aids this is probably my favourite. Activating this option de-clutters design view completely, it takes you right down to the bare bones and really lets you get to grips with the structure of the layout. If I am having a problem with a layout this where I go to, it just makes everything very easy to see. Background images are removed and all your layout elements are given random background colours, it is uncanny how this de-cluttering makes everything so easy to read in design view.
The Dreamweaver Built-In CSS Help Features Series:
Dreamweaver Built-In CSS Help Features - Part 1: CSS Layout Outlines
Dreamweaver Built-In CSS Help Features - Part 2: CSS Layout Box Model
Dreamweaver Built-In CSS Help Features - Part 3: CSS Layout Backgrounds ...