Cascading Style Sheets is a technology that has been around for many years. Cascading Style Sheet (CSS) can assign many rendering properties to HTML elements. Style rules can be specified for a single element occurrence, multiple elements, an entire document, or even multiple documents at once. It is possible to specify many different rules for an element in different locations using different methods. All these rules are collected and merged when the document is rendered to form a single style rule for each element.

The CSS Validator validates CSS style sheets or documents using CSS style sheets.

Uses of CSS
It allows a greater amount of layout and display control than has ever been possible thus far in HTML. The amount of format coding necessary to control display individuality can be greatly reduced through the use of external style sheets which can be used by a group of documents. Multiple style sheets can be integrated from different sources to form a cohesive tapestry of styles for a document. Style sheets are also backward compatible. They can be mixed with HTML styling elements and attributes so that older browsers can view content as intended.

• CSS is much more flexible
• Styles can be written in one place and assigned to HTML elements through class or ID properties. It’s way easier and quicker to change styles across a whole site when they are defined in one place.
  CSS can control styles for multiple documents at once.
• Its classes can be created for use on multiple HTML element types in many documents.
• CSS selector and grouping methods can be used to apply styles under complex contexts.
• It is useful for small quantities of style definitions.
  CSS can override other style specification methods at the local level so only exceptions need to be listed in conjunction with other style methods.
• CSS keeps your style definition separate from your content and content-structure that makes it possible to re-purpose the same content for different media.
• This includes styling pages differently for printing, as well as other user agents like voice (text-to-speech) and mobile devices.

Cascading Style Sheets (CSS) allows you to create fast loading pages, boost your search engine rankings, and modify your whole site with one style sheet. This is because they got so used to html design and are afraid or too lazy to upgrade their skills. Some will also use ready made templates that contain flashy graphics, bloated code and sometimes even contain hidden code embedded in the page.

External style sheets allow you to define a protocol that can be shared among many HTML documents. This is useful in that making a change to the style sheet will automatically be enforced in all of the HTML documents that reference the style sheet.

External style sheets have an extension name of .css and contain a list of rules that are to be applied to any HTML document that references it.