# What is CSS selector? A CSS selector is a string or pattern which is used to select the element which you want to perform style. It is a part of CSS ruleset. 1- Element Selector 2- Id Selector 3- Class Selector 4- Universal Selector 5- Group Selector # Why do you need CSS preprocessors? CSS preprocessors like Sass, Less and Stylus to do best practices like CSS compression and concatenating is an easier manner.