Level 21Go through your checklist. Soon I will stop saying this, as it will become a "goes without saying" thing. There is a lot of info given to you here, the title, even the extra green text. Have you been checking everything in the source this whole time? When looking at the source, it's easy to just skim and look for green text, but it's important to look at anything else that may be important. You've had to mess with stuff in there before, so just remember what you did there.
After that, a bit of work will get you your answer. A lot of levels require mental leaps, but this one is a bit more hands on. Figuring out what the symbols in the picture are would be useful. The trickiest part of this level is realizing there is a trap, as the title may suggest, so be sure to pay attention to what you are given to work with, and figure out which ones don't fit.
I think a little push might be in order for the easter egg. Anything else you can change in the URL besides the .htm name? ͡° ͜ʖ ͡°
Level 22There are times when you must think, and times when you must try the obvious (though sometimes in Notpron, it's good to do that anyway). Even though you may get the answer quickly, it might be fun to try and find the easter eggs.
Level 23As you may have realized by now, the puzzle went through many "temp-ends" every time a new batch of levels went up. Level 22 originally had a more difficult solution, so it was a point of honor to reach this point for many people. You also had to actually call the creator on his phone, so I can understand why he probably changed that fairly quickly, though his number is available on the Notpron website.
The key is in the title. Keep in mind if your answer is something microscopic, that isn't something you can actually see. You might be able to solve this without that hint, but it would be unlikely if this is your first time going through the game since what you are looking for is obscure.
Level 24Compared to previous levels, this might seem like a dry desert of no hints. But there are a couple things to go off of. The title isn't of any help, toss that to the side. Sometimes all it takes is one hint and knowing what to do with it. There is a Google bar for a reason on this one, since the answer could be figured out manually, but it would take a bit longer.
Level 25The source code is only important if it's got green text or other stuff you can mess with, ignore the large block of letter vomit, as that's for the page coding. This level was the first level that I gave up on for a very long time before coming back to years later. The reason for this isn't because it's hard, I just didn't have the tools necessary. However, 10 years later, that tool(s) are readily available to the normal person who owns a computer. Times were different back in my day! *shakes cane*
Just to eliminate any question of what this level requires, you need an image editor, and MS Paint isn't going to do the trick this time around. The original intent of Notpron was to allow a player to get through all the levels without requiring any programs that would have to be purchased separately, so Photoshop is out of the question in that regard. But apparently David M's solution for this one was "just download the trial version and use that". I'm not sure if that's still an option, but it doesn't need to be. Check the sidebar for GIMP, a free image editor that can do all sorts of things, and it's perfectly suited for your needs.
Now what do you actually do to this picture? The page does pretty much flat out tell you "multiply OR limit channels". If you're having trouble finding that, you can look under the "Levels" menu. If you've never used the program before, you may have no idea what that means and have to do a bit of research--but that's the fun of Notpron! Learning new things is a big thing in this game, and this is just the start. You'll feel like a pro fairly soon.