Very solid!
There was some visual buggyness at the start when going in and out of full screen, but nothing unsolvable with a little refresh of the page.
Once in, I ended up with this game open on my screen for I maybe about an hour going through each level at my own pace and in between doing other things--basically like how I would for other puzzle games I've purchased on steam in the past. I literally forgot I was supposed to rate it during this time, and only remembered about 10 minutes ago. So 5/5. I think the music fades into the background but is not silence, you can forget it's there but it's still putting you in the right mind frame. Visually very clean, I would only make some small changes: a line or box to highlight which row your mouse is hovering on during the level selection(I'm guessing this is more a problem for me with an ultra wide monitor so everything is farther apart) & a label of sorts on the start and end gear which would make that clearer. But this is literally me nitpicking of what last bits could be improved.
This is great, I want more levels, and I'd love to sit down with this on my deck.