As publishing and layout standards for web documents, HTML 4.01 and CSS 2.1 define relatively few user experience requirements for browsers. One of them, however, mandates the ability for end users to switch among a set of mutually exclusive document styles defined by the author. This feature is known as alternate style sheets. Significantly, the end user must also be able to turn off all styling.
Check how it works from the W3C’s alternate style sheets example page (IE8 only).
Source : blogs.msdn.com/ie
