Comment by roywiggins

I mean highly dynamic, entirely frontend sites like these are hard to archive, since you have to really preserve every bit of JavaScript dependency, including any dynamically loaded dependencies, and rewire everything to work again.

And then hope that whatever browser features you rely on aren't removed in 20 years. Flash applets from 20 years ago are usually more self-contained and Just Work if you have a functioning runtime (either the official one or Ruffle)