Hacks

From CSS Standards FAQ

Hacks or filters are ways to make css rules and properties only apply to certain browsers. This can be used to fix bugs or even add functionality and let browser that doesn't support the functionality in question to degrade gracefully. Ironicly hacks often relies on bugs in a css parser and should generally not be trusted due to the fact that in future version of the browser, the filter bug might be fixed, but not the bug the filter fixes. This is why it's recommended to use conditional comments to fix bugs in Internet Explorer, which is not only a "secure" filter method, but also a flexible one.