The change has been a long time coming, with YouTube working steadily to default to HTML5 across the major browsers, including Chrome, beta versions of Firefox, Internet Explorer 11, and Safari 8.
HTML5 is an open standard supported natively across a variety of devices and platforms including PC web browsers, mobile devices, and smart TVs. Flash, by comparison, is proprietary technology owned and controlled by Adobe. Even though Flash was long considered the de facto standard for web-based video, it isn't a native component of most browsers and either has to be enabled with a built-in plugin or a downloadable one from Adobe. Flash, like many browser plugins, also has a long history with security vulnerabilities that put users at risk.
The story behind the story: The end of Flash's dominance for online video, at least with the major companies, is still a work in progress. Apple was way ahead of the pack when it decided it wouldn't support Flash on the iPhone. Google eventually followed suit with Android after a short-lived flirtation with Flash. In the short term, Flash video looks prepared to survive with online advertising, smaller video sites, and YouTube visitors who aren't using a modern browser.
No more Flash embeds
As part of the push to put Flash into the background, YouTube says it is also deprecating its option to embed YouTube videos as Flash objects. The site offers its iFrame embeds instead, which allow YouTube to decide on the fly whether to use HTML5- or Flash-based video depending on client-side capabilities.
YouTube's embed transition has been ongoing for some time, just as its efforts to default to HTML5 video on the major browsers have been. When you try to embed a video from YouTube you'll be hard-pressed to find a video with the option to use a Flash-based embed code--an option that was not the default but readily available just a few years ago.
The YouTube announcement did not say the service was dumping Flash altogether, but you have to wonder how long it will be until that happens.