This is a riddle, not a game with a riddle in it. There are 100 levels. Each one is a page. Each page has exactly one way forward and you are not told what it is.
Most of them want you to use a browser properly: the view-source menu, the developer console, the address bar, and once or twice a terminal. The pages are the clues and the tools are the game.
Only the first address is a number. After that they look like 6251chapter — so there is no /l/100 to type and nothing to walk to. You get each one by getting the one before it.
Progress is kept in this browser only. No account, no server, no leaderboard, nothing to lose but the run.
Before you start
There are three ways forward in this game: you click something, you type a username and a password, or you change the address. Every level uses one of them, and the game never says which.
The number in the corner of each level is always telling the truth. Everything else on the page may be.
Open the developer tools early. Several levels print nothing at all and wait for you in the console.