Let’s see, what do we need to develop a basic game?
- Graphics and animations
- Capture user input (keyboard, mouse)
- Sound output
- Also good: Save game states to local storage
Now with HTML5 you can control sound natively in the browser and use flash as a fallback for older browsers that don’t support it (like IE6) – Soundmanager2 makes it this way for example.
Same thing could be done with local storage (or a socket): Use HTML5 if available, flash as fallback.