Opacity controls the opacity of an element AND EVERYTHING CONTAINED WITHIN IT
(supported by webkit, mozilla, opera)
RGBA affects only the opacity of the background color
(supported by webkit, mozilla, opera)
=> expressed as rgb value PLUS opacity degree set by decimal expression
rgb(55,146,179)
rgba(55,146,179,.25)
PNG with alpha transparency is supported by webkit, mozilla, opera, IE7+
