Cascading  Style  Sheet  : The Style Sheet of HTML

The  power and flexibility of  Style Sheets  is that  the appearance of hundreds of Web pages  can be changed by changing just one file.

 Style sheets separates contents from presentation.

A style sheet is made up of style rules that tell a browser how to present a document. There are various ways of linking these style rules to  HTML documents, but the simplest method for starting out is to use HTML's STYLE element. This element is placed in the document HEAD, and it contains   the style rules for the page.

          Each rule is made up of a selector--usually an HTML element such as BODY, P, or EM--and the style to be applied to the selector.

        There are numerous properties that may be defined for an element. Each property takes a value, which together with the property describes  how the selector should be presented.

        Style rules are formed as follows:

             selector { property: value }

        Multiple style declarations for a single selector may be separated by a semicolon:

           selector { property1: value1; property2: value2 }

        As an example, the following code segment defines the color and font-size properties for H1 and H2 elements:

             <HEAD>
             <TITLE>CSS Example</TITLE>
             <STYLE TYPE="text/css">
             H1 { font-size: x-large; color: red }
               H2 { font-size: large; color: blue }
             </STYLE>
             </HEAD>

        The above style sheet tells the browser to show level-one headings in an extra-large, red font, and to show level-two headings in a large, blue font. The CSS1 Specification formally defines all properties and values available.