Mainstream Sciter now uses QuickJS of Fabrice the Magnificent (https://bellard.org/quickjs/).
QuickJS complies to ES2020 specification. And Sciter exposes most popular and modern DOM API so MithrilJS, PReactJS and other libraries work as they are.
Sciter implements far not whole set of DOM API - just essential ones. In fact DOM API and so browsers are full of garbage - old historical layers of stuff from initial Netscape JS API, IE6, etc. For example there is no XMLHttpRequest but there is Fetch API : https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API
Sciter goal is not to support browser API as it is - with all its bells and whistles, after all desktop use cases require features that are not available in browsers anyway.
But all non-UI libraries from NPM collection should work as they are in Sciter. Most of pure Vanilla JS UI components should also work as they are, for example LeafletJS (maps) https://sciter.com/sciter-js-runs-leafletjs-as-it-is/
Also Sciter provides rich set of built-in components like JSX, Reactor, Virtual List (https://sciter.com/behaviorvirtual-list-for-sciter-and-scite...) and others.