The most important skill or knowledge every developer should learn is these three basic building blocks: HTML, CSS, and JavaScript. You will be using HTML and CSS in the front-end for interfaces. Just right click on your web browser and then select the view page source option. You will find the structure of your website where a lot of HTML tags are used for different purposes.CSS is also used in the front-end that decides the style, design, layout and how HTML elements need to be displayed on the screen.Javascript is high in demand nowadays, and it is basically responsible for making HTML pages dynamic and interactive. Javascript also comes with a variety of languages, such as PHP, Python, and ASP.Net, to make a website more interactive. If you're going to specialize in Javascript or MEAN Stack, or MERN stack, then you're going to deep dive into this language because this one will be your front-end -- as well as back-end -- language.