Contribution Guide
If you have not yet read it, read the main contribution guide.
You can get started with Revolt for Web by following the development guide on the repository.
Contribution Policy
Since this project is still heavily in development, things may change rapidly at little notice, so it's recommended that you join the Revolt server and the Revolt development server where you can talk in #Revolt for Web
about this project.
If something is explicitly marked as 'help wanted', then you can likely just pick up the issue and contribute a PR, however, you should make sure to communicate this on the issue so others don't start on the same issue as you, causing issues later.
Otherwise, feel free to come ask about what you'd like to contribute and run it past a maintainer so that we can check if there are any potential conflicts.