Learn how you can use what you've learned about web development to make desktop and mobile apps.
Not only can you make web sites and web applications using HTML/CSS/JavaScript technology, but you can also make both desktop and mobile applications using these things. Desktop and mobile app development is significantly different than web development so it will require some additional learning.
These will all take quite a bit of experimenting with to get used to. They are usually more complicated than just creating web pages, but easier to learn for those already familiar with HTML/CSS/JS. But, since they are cross platform, you don't have to write a completely different version of your application for each operating system.
If you want to make a desktop app using HTML/CSS/JS then you might want to check out Electron, a desktop application framework that uses Node.js under the hood. The code editors Atom and VSCode were created with Electron, as were many other desktop applications, so it seems to be a pretty solid choice.
Once you create something with Electron you will be able to generate a Windows, OS X, and Linux version of your application with little to no additional work.
For mobile applications the most popular JavaScript frameworks seem to be:
Once you know how to use them you will be able to generate both Android, iOS, and even Windows Phone versions of your application with little or no additional work.