Adobe Flash Player
Steve Jobs also claimed that a large percentage of the video on the Internet is supported on iOS, since many popular video sharing websites such as YouTube have published video content in an HTML5 compatible format, enabling videos to playback in mobile web browsers even without Flash Player.
HTML5 in mobile devices
Offline support helps browsers cache static pages. They depend more on HTTP response headers sent by web servers to fetch HTML, CSS and multimedia required to render the web page. If everything required to render is cached, then a page loads quickly, but even if one item is not cached then everything slows down dramatically.