Level 1
Hints : Title translate to NOT in binary, URL means boolean in 1337

Boolean function NOT basically invert 0 and 1 so :
= 0110100001100101011011000110110001101111
Translate with binary code
→ hello
Level 2
Hints : the 1st element of the sequence ("u") is in black font, inspect source code or just select it

This is a variant of the 1, 11, 21, 1211, 111221... sequence
It works better in french where "un" means one, so you would say one "u", one "n", ...
But anyone can figure out that :
u → unu
n → unn
At the end it will give you : unuunnunuunuunnunnunuunnunu
Level 3
Hints : Title tells us there are 6 steps. (the URL 1518 is used later on)

The cipher proceed like that :

1st step : base64
2nd step : decimal
3rd step : morse
4th step : hex
5th step: binary (or text, depend on which XOR decoder you are using)

You should get 4:"-< or 0011010000111010001000100010110100111100
The key is "01011000 01001111 01010010", which translate "XOR", meaning the last step is a XOR cipher
6th step : 0011010000111010001000100010110100111100 XOR 01011000 01001111 01010010 (or using text value, both works on, the site I linked in the first page hints)
It translates to "lupus"
Level 4
Hints : Title "files - rank" chess terms meaning you should look for letters on specific squares of the chess board (labelled a1, b2, d4, f7,....), see this for more infos.
You would also notice the first 5 letters, they are the only one in caps and have a special effect. They will be used later on.

The squares are given by the fonts (in "font.png") and the numbers. The first letter of the font name is the file, the number is the rank.
This requires to recognize fonts (I used very recognizable fonts that can be found in word, paint, photoshop, ....)
This is how the solution should look like :
The squares are : f8, b6, a7, c4, g6, c2, c1 and f5
→ xyzxclpng
Level 5
Hints : Title "FYEO" and name of the pic "1981bond.png" both refers to For Your Eyes Only. Meaning no editing software is needed, this is a genuine optical illusion
URL as png in it.

If you look at the pic from a distance, gently shake your head and/or look at it from a corner, you will see Gaben.
"Cipher :" in black font just bellow the encrypted text -> use the previous password (and current url), to find the picture
Reverse google will tell you it's an autoclave, hinting for the autoclave cipher.
The encrypted text translates to : toSeeTheEncryptedText/YouHaveToTakeAstep.back
"A Step back" both hints for the optical illusion, and for the real password being in the previous step (aka level 4).
This is where the special 5 letters on the chessboard come in place. (another hint was that both gaben and tiuvb have 5 letters).
Using autoclave again, tiuvb gives you "nitro".
Level 6
Hints : URL "zip.html". "2 keys to break the safe." in source code. "Spectrococcus" as author of the mp3 file.

First key, the lock : the values 10,20,30,... don't really matter, but the length of each move is morse code, if it only advances by 10 it's a dot, by 20 it's a dash. When it comes back to 0 it's a new letter.
The animation gives : .- ... .--. .. .-. .. -. → "aspirin"

Second key, the sound : download the mp3 file and check the spectrogram (Audacity will do that), you will be able to read "ANEMIA".

The safe : change the extension of Safe.mp3 to (hence the URL)
Open the zip file with Winrar. It will give a Vgnr.txt file (hint for vigenere cipher), use the first key as the password to open the file.
Use the second key to decrypt the vigenere: pbbhqruf → poxvirus
Level 7
The trick was making everyone think it's a not_found page. But this one is different, the length of the scroll bar can hint you that.

The name of the picture is nope2.png, meaning it's different from the usual Nope.png in all the not_found pages.
Download both pictures and overlap them (you can use the "difference" option in photoshop, not sure on how to do it in gimp).
Adjusting contrast between the pics can help too.
The password hidden in nope2.png is "SPHEE"
Level 8
Hints : Title "nadrac" → cardan. URL "droab" → board.
All the black pieces you find on this page should be used to cover the chess board from level 4. Rotation is sometimes necessary.
The result should look like this :
(NB : I realize after I took that screenshot that the pieces in the middle can be all rotated by 180° and we would have a mirrored solution, so I changed 7.png so it can only fit on the "i" and cannot fit on the "m")
The order of the letters is given by the pieces' name (1.png, 2.png, ...)
→ aspergillus
Level 9
Hints : Title "jpg? since when do I use jpg?", and the name of the picture "jpg.jpg.jpg", telling us the puzzle has to do with jpg format, and possibly 3 jpgs. In the picture properties : Title "Just Say The Magic Number". Comment "AriTauGemCanLeoVirLibScoSagCapAquPis".

Download the pic and edit it with Notepad++.

All jpg files starts with "ÿØÿà" and then "JFIF" (for JPEG File Interchange Format, the standard format). These are also known as Magic numbers (hint in the picture properties).
If you search one of these terms, you will find 3. And they correspond to 3 separated pictures.
You can make 3 copies of the initial picture, and delete the raw datas of one picture at a time to reveal the 3 pictures.
I wrote just before the 2nd pictures : "Nota bene : you cannot copy-paste the NUL byte on windows. Best is to work directly on this file (or a copy).", just to make sure people don't make the error of copy-pasting the raw datas.

This will give a 4x5 symbols sheet. These symbols are from zodiac cipher (hence the zodiac signs abrev. in the picture properties).
→ the password is lithium
Level 10
Hints : Title "blank"

Nothing really appearing on the page.
Check the source code, select where the puzzle usually is (before the script part).
The number of blank spaces corresponds to a letter using a1z26.
→ haemophilus
Level 11
Probably the most tricky, no hints, nothing.

Absolutely nothing on the page or in the source code.
EXCEPT! The password box and submit button. So that's the only thing you can work with.
If you take a closer look at the submit button, and change its contrast (either by tilting you screen if you are on laptop, or downloading the button, since it is actually a picture! (right click on it, inspect element, style part, you should see the url of submit.png)).
You will see : "The password is : pemphigus"
This has been sitting here during most of the puzzle chain.
Level 12
Hint : URL "Final Halt" for alt code, "base32" in source code, and I hinted where it should be a number and not a letter with "#" and when you should use "decimal"

This last level includes almost all the previous levels.
The first number is the level.
Then the other numbers translates with alt-code (holding alt on your keyboard and typing the number on the numpad), it looks like decimal code but dec doesn't work as well.
Level 3 : url → 15 18 → a1z26 → or
Level 12 : title → ... → morse code → s
Level 4 : ♥ ◄◄▼◄▲◄▲►►▲▲▲◄ (go to the chess board, look for the heart (middle right, adjust contrast if necessary) and then follow the direction) → x
               c4 →→↑→↓↓→→↑↑← (same thing but from c4 square) → e
Level 1 : < ! - - → look for that in source code → at line 3 → 3
Level 9 : nbsp → count the number of nbsp → 10 18 14 → a1z26 → jrn
Level 5 : .glitch → in the .glitch section (style), mind the spaces (color, font-size, position, ...) → 01111010 → z
Level 6 : length → 50 lines → 50 in decimal code (or alt code) → 2
Level 7 : XMP¬ → look in the metadatas of the ¬ shaped piece → x
Level 2 : UTF-x → x is for 7 → a1z26 → g

After all that you should get : orsxe3ljnz2xg
Translate that with base32
→ term1nus
And that's it!
(oh and for the raffle, the password is the last word (check source code on the last page) → confucius)