Styles in HTML 4.0 define how HTML elements are displayed, just like the font tag and the color attribute in HTML 3.2. Styles are normally saved in files external to your HTML documents. External style sheets enable you to change the appearance and layout of all the pages in your Web, just by editing a single CSS document. If you have ever tried to change the font or color of all the headings in all your Web pages, you will understand how CSS can save you a lot of work.
CSS is a breakthrough in Web design because it allows developers to control the style and layout of multiple Web pages all at once. As a Web developer you can define a style for each HTML element and apply it to as many Web pages as you want. To make a global change, simply change the style, and all elements in the Web are updated automatically.