Why use CSS

Complete CSS Guide: Why use CSS?

Why should I use CSS?
Style sheets exist, above all, to enable the following principle to be put into practice.
Web pages should separate content from appearance.
As a developer this means that the information in your web site should go into your HTML files, but HTML files should not contain information about how that information is displayed. And you've probably guessed by now that information about how the pages should appear goes into CSS files.
You might wonder what advantages this conveys. Why go to all of this trouble? Just a couple of advantages might give you an idea about why this approach has long been considered beneficial in areas of document management that have been around a lot longer than the world wide web.