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The "CSS3 Color & Opacity" Lesson is part of the full, Introduction to HTML5 and CSS3 course featured in this preview video. Here's what you'd learn in this lesson:
CSS is a consolidation of 50 modules being simultaneously developed and updated rather than one long doc http://www.w3.org/Style/CSS/current-work#CSS3 . CSS3 color and opacity uses background as primary and overlapping color with some level of opacity – widely supported, though not with IE6. Use the filter property to use opacity in IE6, changing the RGBA value to a hex value.