YouTube’s Engineering and Developer’s blog:

Four years ago, we wrote about YouTube’s early support for the HTML5 <video> tag and how it performed compared to Flash. At the time, there were limitations that held it back from becoming our preferred platform for video delivery. Most critically, HTML5 lacked support for Adaptive Bitrate (ABR) that lets us show you more videos with less buffering.

Over the last four years, we’ve worked with browser vendors and the broader community to close those gaps, and now, YouTube uses HTML5 <video> by default in Chrome, IE 11, Safari 8 and in beta versions of Firefox.

YouTube was one of the first major supporters of Flash and one of the last to move on. Flash is not supported at all on iOS. As to Android, there’s this from Adobe’s Flash Player Help document:

Adobe Flash Player is no longer available in Google Play Store for downloading. However, Android users can download and install Flash Player from the Archived Flash Player versions page. This document provides the steps on manually installing Flash Player on Android devices.

And now, the Flash team must be thinking, “Et tu, YouTube?”