Yeah I remember CodePen refreshing for me plenty of times.

No autocomplete involved there.

Well, except for the cases when you happen to have the entire VM for your language running in the same tab where you program lives, and they both are all JS.

Probably harder, and wouldn't work equally well for everyone, but it would be possible to have "guess language and autocomplete" entirely client-side, especially when it comes to JavaScript of all languages.