A relative told me that iOS can playback YouTube videos.
I thought that YouTube requires Flash. How come this is possible without Flash in iOS?
YouTube videos are offered in multiple formats, not only Flash. If viewing on an iOS device, then you get H.264 format.
Safari will show what the developper of the given website provides as alternative content for browsers (user agents) that do not support Flash. In many cases this is just nothing or the hint to download Flash player plugin. Youtube videos are played by the built-in youtube app of iOS.
There is a Youtube app built in which does not require flash. It probably makes use of HTML5 markup.