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 HTML and JavaScript . CSS usually styles HTML elements , but can be also used with other markup languages like SVG or XML .

A CSS rule is a set of properties associated with a 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.