Very fun game, I love platforms with good controls, and this one nails it, but I did found some issues, so here's my honest feedback.
I got stuck inside the right wall in level 6, and the ability to wall-jump, that i guess wasn't intended, makes levels like the 11 a bit trivial. Most importantly, when non full-screen, parts of the level are not visible. I'd say to at least make some warning in the description of the game. And lastly, the ability to restart the level immediately would be most welcome: sometimes you just have to wait so that you can try the level again.
By the way, I'm no artist, but i think that a little bit of work could improve the game visually drastically. If you want, check how I did the portals in my game. They are just a black circle with a particle system on top, but i think they look really nice