But I was told there would be no math! Too bad.
|--MACINTOSH TECHNICAL NOTE #31|
It's a common thing, at least in print design, to use shaded variations of a background to make portions of the design stand out. A good example is an ad in which there's a big picture of a mountain or beach or beautiful woman filling the entire ad, and in the middle is some compelling yet meaningless text, and surrounding that text is a region where the picture in the background has been washed out, as if the text were written on a half-opaque block of plastic.
Since opacity styles aren't part of CSS as of this writing, it's been generally thought that such effects are effectively impossible. There are fixed-attachment backgrounds ...