CSS stands for Cascading Style Sheets. It is a style sheet language used for describing the presentation of a document written in a markup language like HTML. CSS is used to define styles for your web pages, including the design, layout and variations in display for different devices and screen sizes.
Example:
body {
background-color: lightblue;
font-family: Arial, Helvetica, sans-serif;
margin: 0;
}
h1 {
color: navy;
margin-left: 20px;
}