YouTube Allows browser to cache videos
YouTube added a new feature that allows your web browser to cache the videos — just go to a YouTube video, wait until the video is buffered and then reload the page. You'll notice that the video starts instantly and you can watch it directly from your browser's cache. This works even if you close the tab and open the same page later. You may want to increase the size of your browser's cache to preload longer videos.
YouTube had disabled caching probably due to copyright issues.