There is no doubt that JavaScript is the King of web development and probably the most popular language among web developers. It’s also the only language that allows you to create web applications, both frontend, and backend as well as mobile applications
Python is another language which has gained a lot of popularity in recent year. While it's not exactly new and has passed the test of time, it's probably the most versatile programming language at the moment.
It’s a dynamic, server-side scripting language that makes it really easy to create fully functional web applications. If that’s not enough, half of the internet is running on PHP’s shoulder, remember WordPress, the most popular web application software is made on PHP.
Ruby is another programming language that is really great for web development. Similar to PHP and Python, Ruby is also easy to learn and goo for beginners. What makes Ruby special for web development is the Ruby on Rails framework which powers websites like Github, Shopify & Airbnb, Groupon.
While HTML and CSS is not really a programming language (well, some of you may not agree but yes, they are not the programming language) but they are the backbone for web development and every web developer must learn them.