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Cascading Style Sheets (CSS) refer to the language which is used to design the layout and presentation of a web document created in HTML, DHTML, XHTML or any other type of XML document. Via CSS, the designers can define the fonts, colors, styles and overall layout of a web page. CSS improves the web designing work by reducing the complexity. They provide more flexibility and control over the presentation of characters and their style on a web page.

CSS acts as the base platform which is responsible for the format and layout of an HTML document. It allows the same HTML content to be viewed in different styles for different purposes like for print, for web viewing etc. CSS are very useful when the structure of the HTML document is repetitive. For example; if you are making an HTML document in which you have used <Font color=red face=comic sans ms> a lot, then instead of writing this code again and again a CSS can be created in which you can define the type of font you want. This way all the tags and their attributes which helps in designing the appearance and style of the web page such as background styles, borders, character size, heading styles and many more can be defined using CSS. The main advantage of using CSS is that for a single HTML document, multiple CSS can be used. In this way the appearance and layout of the web page can be changed whenever needed in no time. CSS can be used as a separate document or it can be embedded in the HTML.CSS can be created by different people in different styles. It is prepared by both the web authors and the users. The web author makes the CSS having different styles which can be applied to the HTML document. A user prepares a local CSS which is applied to all the documents of a website. CSS are also created in ‘user agent style’ which specifies the default settings for a web browser to display the web page.

Advantages of CSS:

1) CSS allows us to create templates for the HTML documents in which we can predefine different visual styles and appearances.
2) CSS reduces the structural complexity of HTML documents.
3) CSS separates the designing part and the content part of a web page. This leads to the better management and understanding.
4) CSS can be used with any XML language.
5) CSS really helps in managing large websites having a number of interlinked web pages. It assures consistency in the layout and design.
6) CSS can be used to view a web page in different styles depending upon how you will be using the final output. For example; for viewing on web the layout of the web page is different and for printing purposes another layout may be needed.

Apart from these remarkable advantages, CSS suffers from some disadvantages too. There are some issues which creates hindrance while using them. The most common problem is that not all the web browsers support all types of CSS. There are some other issues regarding the expressions and their usage in CSS. Thus, developers are finding more tools and techniques which can improve the functionality of CSS in future. The World Wide Web Consortium (W3C) maintains all the CSS specifications.