HTML (Hypertext Markup Language) - HTML is at the top of the list for a reason: it's the most foundational aspect of web development. The best part is that this markup language is incredibly easy to learn. The only web development tools you'll need are a good text editor and some quality tutorials, and you can pick it up in a day or two.
CSS (Cascading Style Sheets) - While HTML is vital for any front end developer, as it provides the structure of all web pages, CSS is a close second in terms of importance. This is because, without CSS, your web pages will look boring at best and, at worst, like something from a true amateur back in the 90s.
Responsive Design - We live in a mobile-first world now. And every year, more and more people are using their mobile devices as their primary means of accessing the web. That’s why it’s so essential to master responsive web design. If your sites or web applications don't look good and function well on smaller screens, it might as well not exist.
UI (User Interface)/UX (User Experience) Design - While many dev teams have dedicated user interface and user experience designers on staff, some employers may still require you do do some UX/UI work yourself. Besides, developing quality sites and apps becomes much easier when you understand at least the basics of these fields. While you might not find yourself doing user research or other more involved aspects of user experience and user interface design, you should still be competent in the basics of how people interact with sites and applications.
Visual Design/Graphic Design - In short, it's awfully difficult to delight clients, employers, or users with an attractive site if you don't take into account the importance of aesthetics and visual design.