After stopping using ugly IE hacks and moving to conditionnal comments to load a special IE stylesheet, I now use conditional comments to mark my body element with classes reflecting the current IE version.
<!--[if IE 6]><body class="ie ie6 ie-lt8 ie-lt9"><![endif]-->
<!--[if IE 7]><body class="ie ie7 ie-lt8 ie-lt9"><![endif]-->
<!--[if IE 8]><body class="ie ie8 ie-lt9"><![endif]-->
<!--[if IE 9]><body class="ie ie9"><![endif]-->
<!--[if !IE]><!--><body class="nie"><!--<![endif]-->
This saves me a lot of trouble : less files to manage and easier fixes to write. I quite happy with this solution and have tested it accross several projects for the past 3 months. It works really well.
I had to work on a legacy project last week, where this technique wasn't implemented but all the css code was still compressed using CSSTidy. And I ran into a couple of issues.
CSSTidy messes the star and underscore hacks
Using the brilliant _property and *property hacks to target IE6 and IE7 does not work in conjunction with CSSTidy.
For the _property hack, the property is kept as-is, with the underscore, but as they are alphabetically arranged, the _background gets added before the background, rendering it absolutly useless.
On the other hand, on the *property, the * gets removed, and the value is merged with the original value of the correct property. Useless too.
Other solutions that worked
To avoid digging into CSSTidy one more time, I tried to find other ways to achieve the same effect.
To target IE6 I used the !ie6 hack by writing .mySelector { property:value !ie6; }. IE6 is dumb enough to understand any !blahblah as !important.
I could have also used the fact that IE6 understands .class1.class2 as .class2, and could have written .ie6.mySelector { property:value; } (of course, you have absolutly no class="ie6" in your code)
To target IE7 I made a custom selector that its parsing engine is the only one to understand : *:first-child + html .mySelector { property:value; }
