What are cascading style sheets (css)?
CSS is a language used to describe formatting of a web page or other content so that it can be rendered, or displayed, correctly by an user agent.
The Cascading Style Sheets for your web site will define the look-and-feel of your web pages. This includes font usage, positioning of elements, and the sizes of text and other components of your web site.
While HTML defines the relationships between pieces of content, the cascading style sheets tell the web browser what those pieces of content look like.
CSS is one of the standards web designers and web developers are supposed to adhere to when building web sites to insure browser compatibility, usability, and accessibility.