The CSS2 Opacity property doesn't work in the current versions of Internet Explorer - through IE8. However, it has provided a way to achieve similar results with a different method, using IE's filter property. Javascript frameworks usually work this in for you when you try to set an elements opacity in a cross-browser fashion.
It should come as no surprise that there are bugs that arise from this filter property. One is that the filter doesn't cascade to children that have their position as anything besides static.
