Most of the technologies you use when developing Web applications and Web sites are designed and standardized in W3C in a completely open and transparent process.
In fact, all W3C specifications are developed in public GitHub repositories, so if you are familiar with GitHub, you already know how to contribute to W3C specifications! This is all about raising issues (with feedback and suggestions) and/or bringing pull requests to fix identified issues.
Contributing to this standardization process might be a bit scary or hard to approach at first, but understanding at a deeper level how these technologies are built is a great way to build your expertise.
If you're looking to an easy way to dive into this standardization processes, check out which issues in the W3C GitHub repositories have been marked as "good first issue" and see if you find anything where you think you would be ready to help.
Another approach is to go and bring feedback on ideas for future technologies: the W3C Web Platform Community Incubator Group was built as an easy place to get started to provide feedback on new proposals or bring brand-new proposals for consideration.
Happy Web building!