CSS is part of the Web Standards, you can see the entire structure of the Web Standards here
"Web standards are intended to be a common base... a foundation for the world wide web so that browsers and other software understand the same basic vocabulary". Eric Meyer
These 'Web Standards' are designed to:
deliver the greatest benefits to the greatest number of web users
ensure the long-term viability of any web document
simplify code and lower the cost of production
deliver sites that are accessible to more people and more types of Internet devices
continue to function correctly as browsers evolve, and as new devices come to market
For web designers and developers, Web Standards are about using standards (Structural, presentational, Object and Scripting languages) and best practices (valid, semantic and accessible code) to benefit your users, your clients and yourself.
Quite often, a company needs to manage several kinds of Web sites — usually an external Web site (Internet), one or several internal sites (intranet) and sometimes, B2B sites (extranet). Within each of these Web domains, individual sites can be very different depending on their function and purpose. For example, it is common to see combinations of corporate sites, marketing, customer support, portals, knowledge bases, forums and online applications. In large international corporations, one can count hundreds, if not thousands, of sites — often managed by different teams. These teams deal with the following issues:
corporate style guide (consistency in image and branding)
content management (creation, publication, maintenance)
design and development (methods, tools, in-house team work and/or agency management)
information technology (support, technologies, infrastructure)
All companies are organized differently; in one company, the Webmaster role might be attached to the marketing/communication department; in another, the same role might be managed by the Information Technology (IT) department. Therefore, we will endeavour to focus on the benefits of Web standards in relation to the above issues rather than to a specific organizational role.
Deliver to multiple user agents. The same piece of marked-up content is readily deliverable in a wide variety of browsers and other devices (or user agents, to use the official term), such as browsers, personal digital assistants (PDAs), cell phones, and devices that read text for the sight impaired. Just create a different style sheet for each use.
Improve performance. Pages are much lighter (smaller in size) and therefore download faster, because your content only needs minimal structural markup. We can now replace all of the presentational markup we used to load onto the tags in every page of a site with a single style sheet. As you will see, a single style sheet can define the presentation of an entire site and the user only needs to download it once.
Serve all browsers. With a little effort, you can have your pages degrade nicely in older browsers, so all users get the best experience possible with their available technology.
Separate content and presentation. You can modify, or entirely change, either the content or the presentation (read: design) of your site without affecting the other.
Build fluid pages. It's easier to code for varying quantities of dynamic content within your pages. For example, it's much easier to create pages that can accommodate however many items appear in a given listing or menu of your e-commerce store.
Confirm your code is good. Validation services for XHTML and CSS can be used during development to report instantly on errors in your coding. This provides faster debugging, and the assurance that a page is truly completed when it both displays correctly on screen and passes validation.
Streamline production. Production is more efficient. It's too easy for you (the designer) to be side-tracked into content management, because you are the only person who knows where the content goes in the mass of presentational markup, and so you end up being the one to add ita tedious job and probably not what you were hired to do. By adopting standards-based practices, you can provide simple markup rules to the content team and work in parallel on the presentational aspects, knowing their content and your design will marry seamlessly down the line.
Distribute content more easily. Distributing your content for third-party use is much easier because the content is separate from any specific presentation rules, and in many cases, simply not feasible otherwise.
Make it accessible. It's easier to make your site accessible and meet legal requirements such as the Americans with Disabilities Act, Section 506, known colloquially as ADA 506.
Do less work. You write less code and it's a whole lot quicker and easier to get the results you want and modify your work over time.
One of the powerful aspects of CSS is that content can be re-purposed to suit your needs - without changing a line of html code.