When you fork a Github project, you'll have to examine the code to see what it's doing and how.
With HTML and CSS, it's mostly visual but you can find improvements on semantics or simplifying what's there. If you can improve performance by rewriting a javascript function into CSS, such as animations, submit a pull request with your suggestions.
With Javascript, look for bugs and improve the code that's written. Javascript easily becomes a mess and is often referred to as spaghetti code when it becomes hard to manage.
Overall, look for ways to simplify the code, even if it means more code overall although usually it means less code.
Two examples:
For Github: How could a newbie ever improve the code of someone who's experienced enough to create a library or framework?