Project 4. Positioning in the Background
But I was told there would be no math! Too bad.
?span class="docEmphBoldItalic">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 (see Project 11 in Eric Meyer on CSS for more details), but they aren't supported by Explorer for Windows. One can use semi-opaque PNG graphics, but they aren't supported by Explorer for Windows. In fact, short of hacking the browser with proprietary behavior scripts, there's only one way to get a smooth translucency effect in Explorer for Windows, and that's by manipulating the position of background images.