Color Opacity and Transparency
So far, we have outlined the basics of color value notations, and you could be forgiven for thinking that simply telling the browser what color you want is all there is to it. But with the advent of CSS3 comes an additional level of controlling colors, bringing us the ability to layer semi-transparent or opaque effects onto objects.
which is perhaps the trickiest to make cross-compatible due to all of the browsers and versions that need supporting. While opacity appeared only in CSS3, it has quite a history of early browser support, especially in Internet Explorer (which used the proprietary DirectX filter property). Getting the mix exactly right is a challenge, and some people use different blends to maintain the effect.