I can't really reliably fix the sensitivity of the aiming, but I can make the background brighter! It appears html5 resets the blend coefficient of all active objects to 0, but the problem is easily fixable and the solution will be presented in the next patch.
As for the fourth phase/entity, I may have been a little unclear. The first phase has two movement sets. When you begin, the boss is on a track going around the outside edges of the room, shooting at you. After you reduce his health by half, his moveset changes and he begins to actively follow you. I counted this as a phase, but it's more like a bit of a small shift in the first phase. Sorry if that was a bit confusing. :)
As for the space bar, I'll fix that the same way I did in Spiderbite: changing the button used in the next patch.
Thanks again for your bug spotting! :)