Viability: Your games so far feels like a great concept. I would love to see it have a downloadable version someday, as I think I was getting a little browser lag. I also think that grapple is almost there physics wise but could be improved a little. Maybe a faster tether line. Or maybe a circle that tells you when you are trying to grab something out of radius. Like a red circle that only appears when the mouse is too far away.
Presentation: Minimalistic but quite good. Things look clean and approachable. The map is easy to read, and I know where I can grab, where I will die, and where I'm going. I like how the character flips around and the cute swingline.
Theme: Feels like a platformer where you try your best to not die and get to the end as fast as you can. Maybe add a timer, that documents how fast a level was finished in.
Engagement: You got me hooked trying to go as fast as possible. Good job.