TL;DR - Learn HTML, CSS, JS, they are the tools of the past, present and future.
Web development is a big thing nowadays, and probably would be in the next coming years. Web development is the field of writing applications that would run and accessed via the web browser. The web development field had gone (and is going every day!) through tremendous changes. From the very first days of the internet and to our present days, the web technology advances in huge steps, making state of the art technologies obsolete, creating new technologies almost on monthly basis, and attracting more and more developers, companies and industries to the web development area.
What is a web development really?
In web development we build web applications. A web application is an application that is rendered on the browser, and accessed by the user through the browser. Simply put - when you order tickets to movie in your home town theatre, when you check your bank account balance, when you write a post on Facebook or update your resume on LinkedIn, all of these and more are examples to web applications. You open a web browser, go to a specific URL, and get a full working application. Today you can even find sophisticated applications on the web, like sound and video editing applications, drawing, recording, gaming, word processing, you name it. They're all on the web, and all are accessed via the browser.
So, how do I build one?
What is HTML?
HTML (stands for HyperText Markup Language) was invented by Tim Berners-Lee in the late 1980's. It's a standard that contains data to be rendered on the browser, and directions of how to render it. The browser reads HTML document that tells it what to show and how to show it. For example, we can tell the browser to show the sentence "Hello World", and if we want the browser to make it bold, we'll add HTML directions (called "markups") that would direct the browser how to render the text. In this manner, html contains the data (What to be shown) and the meta-data (how it would be shown).
What is CSS?
HTML comes with very narrow set of abilities to style the page. For real styling we use CSS (stands for Cascading StyleSheet). CSS contain rules that control the page styling and actually can do anything - we can change font family, size, decoration, we can change background colours, images, shadows, gradients, every type of styling is possible with CSS, and more features are added all the time.
But hey, people really build web apps using those three tools?
The answer is yes and no. HTML is too complicated, css is a mess and JS code tends to be unmaintainable very quickly, so you can't really write a web app that is more than trivial and keep your sanity.
Over the last decade tons of tools were invented to overcome those three friends' disadvantages, and modern web applications are written in variety of modern, new and shiny tools. You probably heard some buzzwords like Angular, React, Vue, Meteor, Bootstrap, Node and so on. To be a web developer you will probably have to learn one (and usually much more than one) of those tools.
But here is the trick - every platform, every modern tool, with no exception at all, works on the same basic tools - HTML, CSS, JS. No Exceptions, and I mean it. That means that you just can't start learning how to develop web application without them. If you try to start learning Angular for example, without any knowledge in the three basic tools, you'll get nowhere. So the conclusion is that you have to learn the basic tools and to use them as a jumpstart to other newer tools.
Another advantage for learning the basic triad
Even if you somehow manage to learn reasonably enough one platform without the triad, you are not safe! As mentioned earlier - the web development world is crazy and moving fast. I mean very fast. New platforms are invented on day to day basis. New libraries, new tools. AngularJS had started in 2010, but now it is version 6 and the Angular paradigm had changed drastically during this time. Vue.js was presented in 2017, and as to the time I'm writing those lines, there is some trend to migrate to Vue.js from other platforms. And it's not the end. One just can't keep track of all the new ideas and innovations with this influx of ideas. What I'm saying is that if you don't know the basics, you'd be irrelevant the minute a new technology appears. If you knew only AngularJS, what would you do when version 3 came along, and it's different from the one you know? But since the basic triad - html, css and js - stands on the basis of every platform, when you know it you can migrate to any other new and shiny platform.
In the bottom line - learn html, css and js if you want to be a web developer. They are not going anywhere anytime soon.