CSS hack

From [CSS]:

A 'CSS filter' is a coding technique that aims to effectively hide or show parts of the CSS to different browsers, either by exploiting CSS-handling quirks or bugs in the browser or by taking advantage of lack of support for parts of the CSS specifications. Using CSS filters, some designers have gone as far as delivering entirely different CSS to certain browsers in order to ensure that designs are rendered as expected. Because very early web browsers were either completely incapable of handling CSS, or render CSS very poorly, designers today often routinely use CSS filters that completely prevent these ancient browsers from 'seeing' any of the CSS. For example, CSS filters are very often put in place to exclude Netscape 4.xx and Internet Explorer 4, ensuring that only more recent 'v5' browsers 'see' the CSS.