Sometimes to make advances in software one must break backwards compatibility. Nowadays, Internet Explorer 6 (IE6) is a dinosaur of a browser. It's slow, non-standards compliant, lacks features and is a pain to develop for. As web developers, time constraints and other considerations frequently mean it's just not worth putting in the extra development effort to support it. In those cases, we will want to detect those visitors using IE6 as their browser so that we might explain that it is not supported and offer an alternative. Detecting the browser being used is known as "browser sniffing", and there are a number of methods that can be used.