Update: Conditional-CSS is no longer being maintained. Browser support for basic CSS is now excellent and the situation that called for Conditional-CSS to be developed is no longer present in modern web development. If you require a CSS preprocessor please use SCSS or LESS.
Conditional-CSS allows you to write maintainable CSS with conditional logic to target specific CSS statements at both individual browsers and groups of browsers.
Undoubtedly you will have found many a situation where different web-browsers require different style statements. Conditional-CSS addresses this by letting you add Internet Explorer style conditions inline with your CSS statements. For example: