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I recently came across rgba colours which are a very simple and logical way to produce a colour (rgb) with alpha transparency as well. This is ideal if for example you want to create a translucent white box to put some text in.

CSS does have some (dodgy) opacity controls, but they set everything as transparent including any text or pictures contained within the element. By using rgba, we can set a translucent background colour and have opaque elements contained within.

As usual, to get it to work in IE requires some extra effort, however this hack mentioned here and originally credited here seems to offer a solution.