How about contributing to an open source code base for a backend or frontend web framework? Since a lot of these projects are built with scaling in mind and have often reached complexities far beyond the capabilities of a singular developer to write himself, learning how they work in depth can give you the opportunity to significantly "level up" your skills in an area adjacent to HTML/CSS/JS.
If you don't know where to begin, you can start by reading the issues and pull requests for the project on GitHub, where there is often a discussion of how to solve problems from multiple different perspectives. When you've lurked around for a while and feel like you have something to say, consider commenting and starting communication with other people working on the project. When you hang around long enough and even start making pull requests of your own, you'll have newfound expertise and new contacts to ask questions who might already be experts in the technology you wish to become an expert in.