Internet Explorer doesn't support the following:
So that browser is pretty useless to properly view the opacity example page.
This is what the button bar looks like in IE (and no mouse hover interaction is possible):

The above was a static image, below is a copy of the actual button bar code:
IE can be told to show content with an opacity filter using an IE-specific CSS style, see below. But there are still problems: 1) you'll sometimes get an active content security warning popup, 2) :hover is not possible so the buttons are not highlighted on mouseover. Pretty useless.