CSS is a langiage that works with HTML to design the look of a website or webpage. Cascading Style Sheets (CSS) give consistent style to each page in a website.They are called cascading because of the rules HTML will follow in overriding a design opion. Cascading style sheets are found in the head of an HTML document. You can also create an external cascading style sheet in a seperate HTML document and have it link to other documents.