Using relative measurements in your style sheet, you can present your documents to look good on any monitor at any resolution. You have finer and more predictable control over presentation. CSS has been designed to deal with issues which HTML was not. Older browsers will simply ignore your style sheets so they will not break existing pages. The CSS style sheet mechanism allows readers to influence the presentation of HTML documents.
Style sheets allow both document authors and readers to specify rendering rules for HTML documents.
Cascading means that if both parties supply style sheets then the browser will apply both, using further rules for resolving conflicts. Style sheets will allow companies to easily adopt a house look and feel , and this will help give readers a sense of where they are and what they are reading. Style sheets can be pointed to from Web documents via hyperlinks and this will dramatically simplify the maintenance of Web sites. If the company later changes the house style they only need to make changes in one place. The enormous commercial interest in the Web has created an enormous demand for enhanced presentations.