How to block the cursor in Unity so that it does not go beyond the playing field (It works in Unity, but not when loaded into the browser)?

Question

Unity

public void Update()
    {
        if (Input.GetKey(KeyCode.L) )
        {
            
            Cursor.visible = false;
            Cursor.lockState = CursorLockMode.Confined;           
            print("Cursor is Locked!");
        }

    }