In Firefox 4b9, all content with CSS opacity below
1 is hidden in print preview and not printed at all on
actual print, on OS X 10.6.6.
Similar test with RGBA colors instead of opacity.
A paragraph with opacity:1
A paragraph with opacity:.99999999
A paragraph with opacity:.99
A paragraph with opacity:.9
A paragraph with opacity:.75
A paragraph with opacity:.5
A paragraph with opacity:.4
A paragraph with opacity:.25
A paragraph with opacity:.1
A paragraph with opacity:0
A paragraph with opacity:1