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If you are into web developing, then HTML is probably the first thing you should learn. The question is are you more of a front-end or a back-end web developer? Front-end web developers basically create all the user interfaces, designs of the website and to make the website fully interactive. HTML, CSS, JS and other frameworks like Bootstrap and jQuery play a huge role in this one. Back-end web developers basically create server-side stuff like user authentication, preventing SQL injections or other hacking attempts. For this one, you should master PHP, SQL, or other JS frameworks like Node.js and AJAX. P.S. I'm into back-end for some reason.