OK, so I played this demo and it was quite enjoyable. Still it has some problems.

First problem is difficulty curve. At the beginning levels are quite tough and demanding, so I assumed that this will be hard hack. But Boom Boom Castle was a cake walk and boss was super easy.
From that point the game changed from hard to easy difficulty. That's weird. As for final level of World 2 it's slow autoscroll with a lot of emptiness. A little better is inside. As for boss it's the hardest boss in hack. There could be a warning that boss isn't jumpable. Also he shoot projectiles very fast.
Then World 3 gets normal difficulty with exception of Forest Ghost House which is very easy. And I really suck at dodging Chain Chomps. As for final boss of the hack it's the most dissapointing. It's just a copy paste of Big Boo battle from SMW (Donut Secret House).

Second problem is secret exits. I wouldn't call them secret exits. Keys are very easy to find, keyholes are few screens later. At one point in level of World 3, I thought that pipe in front of me (though behind dirt) was actually a progression of normal exit. But it lead to secret exit instead.

Graphic is generally OK. But there were few things that was weird for me.
On the overworld:
Mushrooms on the sticks looks weird.
The transition from light green grass to dark green grass could be better. Especially on the seashore.
In levels:
There are 2 instances in levels (one before Switch Palace and the other in boss level) where appear sign pointing right with Switch Palace Block. It looks weird.
In the Treetop - inside of a tree is in a color of soil rather than wood.

That's all. I wish you good luck in the future.

Looks pretty good.
I may test it at some point, but I just figured I'd leave you a tip:
put <pcenter></center> without the p around the logo to center it.
M A G I C

Weird, it works for me.
It should look like this