But Sciter do not have codecs embedded. Instead

a. it [re-]uses streaming services of underlying OS;

b. has interface for the hosting application to supply its own video streams.

This way some applications use WebRTC as video sources, some use ffmpeg library to provide video streams. Video playback architecture is app dependent usually. Sciter provides to them "video rendering site".

That's idea of Sciter - to reuse underlying OS services as much as possible.

For example Sciter has spellchecking but only if OS provides spellchecking API (and usually they do).