A problem that seems to be a reoccurring theme that people just don't account for - browser upgrades. We can generally see that in browsers like Safari, Fire Fox, Opera, et al. they get better by adding canvas support, CSS3 selectors and other aspects that help make web development more powerful, allowing for better user experiences online.
The problem child is still Internet Explorer. Yes, it does get better with each instance, but people complain about the upgrades because they aren't coding properly.
This means using conditional comments to target your problem browsers and CSS hacks to target others.
Keep in mind, CSS Hacks are jut that - hacks may not always work, and they are **NOT** considered best practice by ANY means. These stay listed because if your site is going to be short term -