The flag is in the HTML source of the page, in a comment, right after the rules code finishes:

I think everyone who solved this already knows the fundamentals of coding and wouldn't check the writeup - so, for those who did check, you should know that the best way to see the source code of a page is to right click the page and either press "Inspect element" (interactive) or "View source code" (easy to Ctrl-F search). You can also use the built-in menus on your browser to do this, such as in the More Tools > Developer tools section inside the 3 dots at the top of Chrome.