It looks like the way itch.io hosts browser games allows them access to a common Local Storage. I doubt that gamedevs would try to mess up each other's games this way, but the 5MB-per-domain limitation would become a problem at some point.

My claim is based on the observation that checking Application->Local Storage in Developer tools while on https://twotinydice.itch.io/forward page shows me barely anything saved in local storage for this domain, but has a second domain https://v6p9d9t4.ssl.hwcdn.net/ that has multiple items from various itch.io games. (Is https://v6p9d9t4.ssl.hwcdn.net/ a CDN used to host itch.io browser games?)

P.S. I'm not a frontend guy, so my understanding above might be flawed - sorry if that's the case.