Given the number of Github projects that want terminal "demonstrations", I'm surprised Github hasn't just provided first-party support for it (with e.g. a fancy renderer for script(1) code-embeds, that would play back the script session by writing regular DOM text nodes; and which would give you transport controls to pause, scrub through, skip around, etc.)
While they're at it, they could also allow embedding of some kind of slide-deck format. Using GIFs for slides has all the same problems. (Hopefully nothing as fancy/complex as PPT; more like a basic-PostScript-profile PDF renderer, that would auto-carousel through the PDF's pages, until you begin interacting with it.)