Nice platformer. It's kinda simular to the platformer i am working on too. Its great but the jump is a bit slow. Keep up the good work

Here are my thoughts how you could improve this prototype:

• At first I didn't know that I was supposed to avoid the triangles. After I died to the first one, I understood they're supposed to be spikes, but it's not clear at the beginning and there's nothing to indicate that they're to be avoided, so it feels a bit unfair and unnecessary to die to it. Same with the water (?), unless you try it and die to it, you have no way of knowing that it'll harm you.
• When you die, the game makes you go right on the death screen and hit the ball. It does not take a lot of time, but it does take some and if you die a lot, it can be annoying to have to do that every time. Small nitpick to add, all the stuff that hurts you is blue in the game world, but on the death screen you're supposed to hit a blue thing to be revived. Bit of mixed messaging there :)
• Ignoring the left arrow at the beginning and going right results in a fall to the void. I would be fine with this if that made you die, but as it is now I kept falling and had no way of continuing to play without restarting the game (reloading the page).
• When I held a direction and jumped, I jumped towards that direction. When I let go of the direction key in mid-air, I expected to stop going in that direction and fall straight down, which is the traditional way of doing it in most platformers.

Otherwise I liked the tight controls and the level design (though it was a very short level). Keep it up! :)

Nice test prototype! I managed to make it to the end of the level :)

Positives:

    The WebGL works well and is smooth.
    Movement physics is good and accurate.
    Level design is good (It made me want to keep trying till I completed it :)

 Suggestions:

    The collision detection needs a little work. The square can go half-way into the triangle before collision is detected.

Overall, though great start and I look forward to seeing more!