It’s in the link at the bottom of the “What is the Google CTF?” section.
All Google would have to do is move the sentence "they consist of a set of computer security puzzles (or challenges) involving reverse-engineering, memory corruption, cryptography, web technologies, and more" from the secondary page into the paragraph that's supposed to answer the question. It would actually be shorter.
Now on the one hand I'd say, ok, its not google's responsibility to try to have websites/non-standard browser configs set ups actually work with their web pages...
On the other hand, I'm sure the kind of security people google is looking for all surf the web with standard browser configs and permissions enabled...
And as the other commenter has pointed out, I know its radical of me, but I would have thought the "What is the google CTF" section should contain text which answers "What is the google CTF".
Now I'm not a (total) idiot, obviously I can debug it if i have to (and if other people are telling me "there's stuff there, you just can't see it").
If i open it in chrome with extensions turned off or a default mobile browser, stuff now appears, so yeah, its got to do with script permissions and the like.
But again, why not just include actual text in the "what is google ctf" section that answers "what is google ctf?", and/or in the beginner's section that explains what's going on, and of all the groups of people you'd build your webpage for, are security people the ones that you want a website that breaks if they block your default scripts/permissions?
Every puzzle here places you in a different unfamiliar situation. The only "outright disqualifying" thing is whether you can figure out something on your own. You essentially failed the first test.