In this specific case (Elm vs. JavaScript) We're talking about JavaScript here, so:

> - ubiquitous things, like data structures

There are a few basic data structures in JS, but almost everything else is in user-land.

> - hard to get right things, like time

Date is a notoriously bad API in JavaScript.

- implementations of specs, like Unicode

We just get a String type in JavaScript which is UTF-16, so...