pre ie8 -ms-filter needed for opacity, being confused with filter

[etetlow]

As was mentioned, but not explained properly and therefore closed, here: (https://github.com/CSSLint/csslint/issues/93#issue-1127736). When using opacity: 0 and covering pre ie8 with -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=0)" we are given the warning:

Also define the standard property 'filter' for compatibility.

Of course, the standard property is 'opacity', not 'filter' so please can this be rectified as it's the only warning on my project and comes up 439 times!