CSS: CSS 2.1 in full + some most popular CSS3 modules like transition, animation, transform, etc.
CSS flex and grid are supported but in Sciter specification, see: https://terrainformatica.com/2018/12/11/10-years-of-flexboxi...
JavaScript: ES2020 specification in full + native JSX and Reactor, see: https://sciter.com/tutorials/reactor-hello-world/
JavaScript runtime:
+ most popular and used JS DOM APIs
+ Node.JS runtime: FS, Net, Process
+ Desktop specific runtime, for example HTML Windows (https://sciter.com/html-window/), components (https://sciter.com/tutorial-learn-sciters-html-components-in...), etc.
See: https://sciter.com/developers/for-web-programmers/