Conditional CSS
It is desirable that browser specific bugs should be targeted and fixed each browser specifically with no impact on the way general CSS is written. If a bug is fixed in a certain browser version, we need a possibility to remove fixes without worrying about browser compatibility.
Microsoft offers to use conditional comments. Thus, you can write a special CSS file for their browsers. In other words, you include a separate CSS file for browsers that requires CSS fixes for a certain project instead of standardizing conditional comments.
It processes like a normal css file. The only difference is that only those browsers which are targeted will include the css file.
With targeting every browser in a separate CSS file is quite a lot of work that sometimes not possible within the constraints of a project. However, such process allows a developer to implement his often good ideas to use css without having to worry about complying to 5 (or more) browsers. 
As you can see advantages outweigh disadvantages of such a decision. It is necessary to look ahead and try to build a standard till CSS remains to be a problem with bugs in browsers. It is necessary to devise a system where the bugs can be addressed without reverting to hacks and tricks.
Reference: www.onderhond.com/blog/work/conditional-css