The decline of Adobe Flash Player seems to be slow but irreversible as the biggest video service online has taken another step in making it irrelevant.
YouTube started offering an experimental HTML5 player years ago, but only through the advancement of the standard and by working with browser creators was the service able to fully switch to HTML5.
The new player takes advantage of Adaptive Bitrate streaming (ABR) which allows YouTube to seamlessly switch between higher and lower quality streams for continuous playback regardless of network conditions. It also allows for live-streaming of content on consoles, devices like the Chromecast and regular browsers.
Of course this also means that watching video on mobile devices got better, especially if you were doing that through a web browser.