Safari does not come with Flash plugin preinstalled, like in Chrome. And we do not recommend installing the plugin as we view Flash as a resource hog and a technology that is on its way out.
Unfortunately Flash is still a die-hard habit, even for major site such as YouTube. Without Flash installed on Safari, you will get a message “The Adobe Flash Player is required for video playback” when viewing a lot of videos in YouTube.
One way to solve this problem without installing Flash is to use the Safari extension YouTube5, which will automatically replace the Flash player on YouTube to use the HTML5 player on YouTube. HTML5 player will play a non-Flash video format that is supported by Safari without any plugin.
When you view video from iOS, YouTube by default will use the HTML5 video player. But when you visit the site from the desktop, the Flash version will always be served first, irregardless if your browser support Flash or not. Why don’t YouTube default to HTML5 video player?