Описание тега css-hack
A CSS hack is a technique which uses implementation bugs in the style sheet parser of a browser family or a particular version(s) of a browser to identify it and apply specific CSS properties, usually for the purpose of cross-browser consistency.
A CSS hack is usually part of a selector string, but can also be part of a media query or a property value. As of March 2012, CSS hacks are available for Internet Explorer, Firefox, Chrome, Safari, and Opera, but are subject to change as browser vendors fix longstanding bugs and standardize experimental features.