I haven't finished the game (yet!), just at the start of the boss. I've found it to be a creative twist on the typical platformer!
With that said, I have been having a bit of annoyance with the way the controls are mapped to the keyboard (I'm using Google Chrome for my browser, by the way).
First, neither the W key nor the Up arrow key work on my computer. The only button that acts as jump right now is the Spacebar. It would be nice for the W key and Up arrow key to work as jump as well.
Second, only one of the shift keys seems to work as dash (Left side; the one closer to the WASD keys), while the other shift key (Right side; the one closer to the arrow keys) doesn't respond (same as the W and Up keys don't respond).
The controls seem to be adaptable for both left-handed (WASD) and right-handed (Arrow Keys) players, which is great. I personally have been playing WASD. As it is, though, my fingers feel all mixed up. My left hand controls my left and right movement (A and D keys) as well as the dash (Left-side shift key), but it doesn't control my jump movement (Spacebar). Instead, my right hand controls my jump movement (Spacebar) and it controls my world-switching (J key).
(I could theoretically use Spacebar with my left hand but then I would have 4 actions on left hand and one on right hand) (Additionally, I could map world-switch to E key instead, but again, 4 keys on one hand).
As it is, I have been using A key, D key, and Left-side shift on my left hand. And Spacebar and J key on my right hand. What I would like to do is to use W key, A key, and D key on my left hand. And Right-side shift and J key on my right hand. This would allow me to do my main movement functions (move left, move right, jump) on one hand, and to do the more unique actions (dash, world-switch) on the other.
(A similar thing applies if I was using Arrow keys. My right hand controls only left and right movement. And my left hand controls dash, world-switch, and jump. Ideally, my right hand would control the main movement functions while my left hand would control the more unique actions.)
The way I see to make the keyboard mapping feel more natural is to enable the W key and Up arrow key as jump controls (while keeping the Spacebar for those who prefer it?) and enable the Right-side shift key as a dash control.
One last thing (which is more my personal thought), it seems like the game was originally intended to be played without the outlines. Calling the non-outline mode as 'hardcore' and the outline-assist mode as 'regular' feels a bit like you're selling this feature (the world switching and the way it plays into the platforming's difficulty) short. Like the outline-assist mode is the, I guess, 'correct' way to play and to play the non-outline mode is unnecessary to the game. In a way, it can be considered so, and there's nothing wrong with wanting to play only with the outline-assist, but it feels to me as though the feature is being sold short. Maybe you could rename the modes so that the non-outline mode is 'regular' and the outline-assist mode is 'assist'? I'm not sure, but I wouldn't want anyone to overlook the non-outline mode because it has been labelled as the hard mode. This is more of my personal preference than anything, so don't feel like you need to regard it unless you want to, I just thought another perspective there might be nice.
I'm sorry if I fumbled my words at all (and for the very very very long comment), please let me know if I need to clarify what I meant by anything (or if I've overstepped in any way)! So far a very fun platformer with an interesting story concept to tie it all in, keep it up!