The theory of Cascading Style Sheets (CSS) is a means to an end: better, more efficient Web site design. In the real world, however, CSS does not provide a perfect, clear-cut path to that goal.To achieve the promise of CSS, working designers have employed a series of workarounds known collectively as hacks. At the most basic level, a CSS hack is a modification to the standard CSS code. Like any deviation from the norm, the use
of CSS hacks has both its supporters and detractors: Some designers feel CSS hacks are an absolute necessity and others are fervently opposed to them.

To figure out why the Web design community is divided over CSS hacks—and which camp you should be in—you’ll need a little background on the emergence of CSS.
CSS Hacks and Filters