Background music can be added.

The current reflection algorithm is a pure brute force. The ray is traced until it hits a mirror, and then the angle after reflection is calculated using inverse trigonometric functions.

Using equation of line and some geometry, this algorithm can be obviously optimised and result in a much smoother gameplay.

The functions for the reflection algorithm are located in scripts/levels.js

If you move a mirror to the bottom right hand corner, game freezes. Here is the preview:

This is a very simple enhancement. You can look up levels.js and add more levels there. Make sure though, that the level is solvable and the intended solution is the best solution.

P.S.: Try to come up with innovative ideas and possibly add more types of obstacles!