I've been working closely with cross-browser compatibility issues for more than 5 years, and during that time, quite a few people changed on the project I'm working on. During this time, I noticed that developers are often only as proficient in CSS as they need to be to get a given job done, even if a person positions themself as a web developer. These issues are most often highlighted when it comes to supporting Internet Explorer.
For those who've never been deeply involved in such issues, I'll explain what makes IE so special and hard to work with. First of all, developers are not often using Windows as their OS on laptops/computers for development. To work with IE, to verify that it works as expected, you have to install VirtualBox with Windows on it.