CSS Overlays: Using CSS Positioning to Overlay Web Objects
http://www.websiteoptimization.com/speed/tweak/overlay/

Website Optimization
2008

Abstract:

An overlay is when one web object overlaps another. Overlays are often used to highlight or draw attention to important items on websites to raise conversion rates. This article shows how use CSS positioning to avoid slicing and dicing your overlays and assembling with tables. Along the way we'll look at the workarounds we used to make the technique work with different browsers (most importantly IE5.x Mac and Safari).