HTML5 vs Flash: General information about both technologies, from an online video perspective.
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Flash is a technology developed by Macromedia (acquired by Adobe some time ago) that was originally intended for creating vector-based Web animations and even full animated Websites; however, it has the downside of requiring the installation of a plugin in order to playback the content created with it. Although requiring the installation of a Web browser plugin, because of the interactivity and spectacularity that was possible to achieve with it, which was not possible with other existing Web technologies, and thanks to its evolution along the time, that added important programming capabilities and, specially, powerful online video features, it soon became very popular and almost ubiquitous on all desktop computers (up to 99% of presence). Its online video playback features, like streaming, hardware acceleration, "universaility" (videos can be played back in any Web browser, on any operating system, with the Flash plugin), etc. turned it into the preferred technology for developing video players that are used in most online video platforms/video portals/WebTVs in Internet.
And the iPhone came out... With the launch of the iPhone, which has the iOS operating system, Apple decided to stop supporting the installation of plugins for viewing Web content. The decision was based, among other reasons ( +info ), on the fact that they preferred to support the new (and open) Web emerging technologies, like HTML5, instead of proprietary plugins (like Flash) which shortened the battery life of mobile devices. Whether with reason or not, the fact is that iPhone became soon very popular and, then, iPod Touch and iPad which use the same operating system (iOS) without Flash support. The popularization of the iOS based devices (hundreds of millions...) caused that most Web sites started to use the new HTML5 Web technology in order to display content on these devices. Without entering into the details of the "technology war" between Adobe and Apple, where Adobe tried to make Flash available on mobile devices and Apple tried to stop it, Adobe decided to, finally, give up and join the support of the new Web standards like HTML5 and abandon the development of Flash for mobile devices. And so then, Android, the most popular mobile operating system today, that supported Flash since early versions, dropped Flash support since version 4.1 (Jelly Bean), in favor of the HTML5 standard.
It is worth noting that, as for today, Flash continues to be the preferred and most used technology for playing back online video on desktop computer Web browsers.
The development of the HTML5 standard, by the WHATWG*, started in 2004 and was accepted by the W3C* in 2007. Finally, the first official draft was published in 2008. HTML5 intends to commit itself to be the substitute of Flash, promising to bring richer but simpler interactive activity for Web, and although HTML5 is an evolving standard, it is currently mature enough as to begin enjoying its features, besides being supported by the main Web browsers on the market. The main support for the new standard came from the Apple side, but other industry giants, like Adobe, Google and Microsoft, have announced they also support it.
Video... Among the new features offered by HTML5, and maybe one of the most important ones, is the native support for video playback without the need of plugins. This is the capability that has allowed it to became the ideal replacement technology for Flash, mainly on mobile devices.
Knowing the importance of HTML5, specially for mobile video playback, WS.WebTV includes, besides the Flash video player, a complete HTML5 video player. The use of HTML5 is ideal to allow the WebTVs to be viewed on Smartphones and Tablets, without Flash plugin, but with HTML5 and H.264/WebM capable Web browsers; for example: iPad, Android 4.1 devices, etc.
WHATWG: Web Hypertext Application Technology Working Group
W3C: World Wide Web Consortium
Wikipedia: HTML5 Vídeo