Hand in hand with JavaScript is CSS and HTML – together they make up the holy trinity of front end web development. HTML (Hyper Text Mark Up Language) is the language of web browsers – what websites are made with. CSS (Cascading Style Sheets) makes them look cool and better than those awful sites from the early days of the web. It’s essential for front end web developers to know these guys inside out, and pretty helpful for back end developers, so you can understand how your server side changes affect the end user.