It depends on you! What to do after learning HTML, CSS, and JavaScript? Solely depend on your interests and the thing which you are passionate about. If you wish to dwell more into only Front-end development, then I would suggest you have a look into major front-end frameworks, like Django, etc. Or if you are inclined more towards the Back-end side of things, then go for PHP, MySQL, Python, etc. to aid you in creating dynamic websites. If you are aspiring for a career in Full-Stack web development, that means you are capable to design a website from both Front-end as well as Back-end then I would suggest you enroll in a Full Stack Developer course from Intellipaat, which will help you build practical skills, that would aid you in becoming a professional certified Full-Stack Developer.
Also, if you are a beginner, watch our YouTube video on HTML vs JavaScript to help you get started with the basics.