CSS (Cascading Style Sheets) is a declarative language that controls how webpages look in the {{glossary("browser")}}. The browser applies CSS style declarations to selected elements to display them properly. A style declaration contains the properties and their values, which determine how a webpage looks.
CSS is one of the three core Web technologies, along with {{Glossary("HTML")}} and {{Glossary("JavaScript")}}. CSS usually styles {{Glossary("Element","HTML elements")}}, but can be also used with other markup languages like {{Glossary("SVG")}} or {{Glossary("XML")}}.
A CSS rule is a set of {{Glossary("CSS Property","properties")}} associated with a {{Glossary("CSS selector", "selector")}}. Here is an example that makes every HTML paragraph yellow against a black background:
/* The selector "p" indicates that all paragraphs in the document will be affected by that rule */
p {
/* The "color" property defines the text color, in this case yellow. */
color: yellow;
/* The "background-color" property defines the background color, in this case black. */
background-color: black
}
"Cascading" refers to the rules that govern how selectors are prioritized to change a page's appearance. This is a very important feature, since a complex website can have thousands of CSS rules.