Cascading Style Sheets, simply referred to as CSS is a style sheet language which is used to format the presentation of a document written in a markup language such as HTML.
The language can be applied to any XML document but it is usually associated with web pages and user interfaces written in HTML and XHTML language.
The CSS language is a cornerstone technology used by most websites nowadays in order to create visually engaging webpages, user interfaces for web applications and for mobile phone applications.
It is designed primarily to enable the separation of document content from document presentation which includes elements such as the layout and the colors and fonts etc.
This separation is required in order to improve content accessibility and provide more flexibility and control in the specification of presentation on he web pages.
It also enables multiple HTML pages to share formatting styles by specifying the relevant CSS in a separate css file, often called stylesheet.css.
CSS also reduces complexity and repetition in the code content. For each matching HTML element, CSS can provide a list of formatting instructions. For example, a CSS rule might specify that all ‘heading 3’ elements should be bold and of a particular font size.
This separation of formatting and content makes it possible to present or display the same markup page in multiple different styles according to devices and for different rendering methods, i.e. on-screen, in print, or voice etc.
CSS is perfect for formatting webpages that need to be displayed in a number of different formats, such as for desktop computers, tablets or mobile devices. It is also possible to have multiple CSS sheet styles and some websites allow the viewer to specify a particular CSS style sheet. When this is the case, the file would be stored on that users computer to enable it to over-ride the original that the website auther has specified.
CSS specifies content on a priority level which determines which style rules are applied to which pages if more than one rule matches a particular page element and this is where the term ‘cascading sheet styles’ comes from.
All CSS specifications are maintained by the World Wide Web Consortium (W3C).