A scary game in a browser tab has one job that a scary game on a console does not: it has to work on a laptop speaker, in a room with the lights on, with somebody else in it. That rules out most of the genre. What is left is the part horror was always better at anyway — not knowing where the thing is.
Quiet Hours is the one built for it. The thing in the dark cannot see you. It can hear you, and that is the whole game: every surface you cross has a volume, running is faster and louder, and standing still is free but costs you the clock. Headphones make it considerably worse, which is the recommendation. The Ashen Gate is the opposite kind of dark — a dungeon crawl where the threat is in front of you and the question is whether you can afford the fight. Shadow Walk is the quiet one: no monster at all, just a light you move and the shapes it leaves behind.
All three open in a tab on a school or office network. There is nothing to install, no plugin, no account, and no chat — which is usually what gets a horror site blocked rather than the horror. If your filter still stops it, that is its call and not something any site can honestly promise around.
One note on age: these are tense rather than graphic. There is no gore in any of them and nothing that chases you with a jump-scare sting every thirty seconds. Quiet Hours is the only one that genuinely frightens people, and it does it with sound.
