The whole page scrolls up and down when I press on the arrow keys.

Nice initial level to your game. Cool character and background imagery. I would enjoy playing your future levels when you have finished your game. :)

i can tell this is a simple project, and honestly, it's pretty good. of course, it needs more improvement. But, it's not that bad

The author didn't put any effort into this so why should I?

Okay… is it just a single level?
It's a fine prototype for a game, but it just doesn't have enough content. You're done within a minute.

Also, I have a few tips for improvements if you plan to work on this further.
– There are no sounds. Games with sounds (and possibly music) are instantly more enjoyable.
– The dystopian background is quite cluttered and the foreground with the player doesn't stand out as much.
– For some reason, jumping on the platform at the beginning of the level restarts the game even though it looks exactly the same as the ground. The actual ground hovers a bit above the bottom end of the screen (there's a small gap).
– The cave exit has thin white artifacts around it, and it looks like you saved it as *.jpg, and then attempted to fill it with a paint bucket, which resulted in the uneven fill.
– There's an invisible barrier at the top of the screen, which makes it harder to collect the topmost coins (you have to jump precisely underneath it)

All in all, it's a good start, and I hope you'll continue to work on this, but I don't really think it's a fun game to play at this stage yet.