Zencoder
Zencoder is the creator of an open-source HTML5 video player, Video.js, built with Javascript, HTML, and CSS. Video.js plays video via native playback in all HTML5 browsers and devices, or through Adobe Flash when HTML5 playback is not available.
Adobe Flash
Firefox (from version 46) rewrites old Flash-only YouTube embed code into YouTube's modern embedded player that is capable of using either HTML5 or Flash. Such embed code is used by non-YouTube sites to embed YouTube's videos, and can still be encountered, for example, on old blogs and forums.