CSS stands for
Cascading Style Sheets. CSS is a stylesheet language used to
describe the presentation or style of a document written in a markup
language like HTML.
CSS is used to define colors, fonts, layout, and other styles found on a webpage. It is designed primarily to enable the separation of the content (written in HTML or a similar markup language) from the presentation and styling (written in CSS).
CSS also allows pages to more accessible, with users sometimes being given the option of over-riding a site’s style sheet to apply their own, usually to make it easier to read (for visual impairments and dyslexia, or for those who just prefer a certain colour combination on screen).