Web CSS design

INTRODUCTION:
Web design is the skill of creating presentations of content that is delivered to an end-user through the World Wide Web, by way of a Web browser or other Web-enabled software like Internet television clients, micro blogging clients and RSS readers.
The process of designing Web pages, Web sites, Web applications or multimedia for the Web may utilize multiple disciplines, such as animation, authoring, communication design, corporate identity, graphic design, human-computer interaction, information architecture, interaction design, marketing, photography, search engine optimization and typography.
Cascading Style Sheets (CSS) is a style sheet language used to describe the presentation semantics of a document written in a markup language. CSS is designed primarily to enable the separation of document content from document presentation, including elements such as the colors, fonts, and layout. This separation can improve content accessibility, provide more flexibility and control in the specification of presentation characteristics, enable multiple pages to share formatting, and reduce complexity and repetition in the structural content CSS can also allow the same markup page to be presented in different styles for different rendering methods.

Sources
CSS information can be provided by various sources. CSS style information can be either attached as a separate document or embedded in the HTML document. Multiple style sheets can be imported. Different styles can be applied depending on the output device being used; for example, the screen version can be quite different from the printed version, so that authors can tailor the presentation appropriately for each medium.
The style sheet with the highest priority gets used to display the content. Declarations that are not set in the highest priority source, will get passed on by a source of lower priority such as the user agent style. This process is called cascading.
Variations
CSS has various levels and profiles. Each level of CSS builds upon the last, typically adding new features and typically denoted as CSS1, CSS2, and CSS3. Profiles are typically a subset of one or more levels of CSS built for a particular device or user interface. Currently there are profiles for mobile devices, printers, and television sets. Profiles should not be confused with media types which were added in CSS2.
CSS 1
The first CSS specification to become an official W3C Recommendation is CSS level 1, published in December 1996.
CSS 2
CSS level 2 was developed by the W3C and published as a Recommendation in May 1998. A superset of CSS1, CSS2 includes a number of new capabilities like absolute, relative, and fixed positioning of elements and z-index, the concept of media types, support for aural style sheets and bidirectional text, and new font properties such as shadows.
CSS 3
CSS level 3 is currently under development. The W3C maintains a CSS3 progress report. CSS3 is modularized and will consist of several separate recommendations, because all browsers do not comply identically with CSS code, a coding technique known as a CSS filter can be used to show or hide parts of the CSS to different browsers, either by exploiting CSS-handling quirks or bugs in the browser, or by taking advantage of lack of support for parts of the CSS specifications. Using CSS filters, some designers have gone as far as delivering entirely different CSS to certain browsers in order to ensure that designs are rendered as expected.
Bandwidth
A style sheet will usually be stored in the browser cache, and can therefore be used on multiple pages without being reloaded, increasing download speeds and reducing data transfer over a network.
Page reformatting
With a simple change of one line, a different style sheet can be used for the same page. This has advantages for accessibility, as well as providing the ability to tailor a page or site to different target devices. Furthermore, devices not able to understand the styling will still display the content.