His previous hack relied on browsers rounding the result of an opacity calculation in order to force a binary outcome (i.e. one of two colours rather than multi

His previous hack relied on browsers rounding the result of an opacity calculation in order to force a binary outcome (i.e. one of two colours rather than multiple shades). Once they improved the accuracy of those calculations this trick was no longer applicable I'm probably not explaining it very well, but you can read the full details of the hack here: http://lcamtuf.coredump.cx/css_calc/ http://lcamtuf.coredump.cx/css_calc/