HTML5's <video> tag should've killed YouTube. Part of the reason why YouTube got so popular was because people were tired of having to install multiple plug-ins just to watch videos. Some websites required Windows Media Player, others required QuickTime, others required Flash, others required RealPlayer. YouTube, using a flash-based video player at the time allowed pretty much anyone to watch their videos because almost every Internet user had Adobe Flash installed.
@PurpCat Never heard of that. And looking it up doesn't show me anything.