The full stack developer should know the following languages. The front end of any website application is the part that the user sees and interacts with directly. The front-end developers usually work with HTML, CSS, and JavaScript. The back-end web developers will work along with the front-end web developer to ensure that their code works within the design of the website and with the front end. The full-stack developers will work with the front end and the back end of the website. They know most of the front end and back end programming languages.