I do not believe Yahoo hosts the videos on their servers. I think they link to say YouTube, or other hosting sites. So it may not tell you whether you viewed them or not.
If you're logged in to YouTube, the History:
stores a record of every YouTube video you have ever watched
while logged in, (including videos watched through links on websites, but excluding videos which have been removed from YouTube). Records of removed videos remain in the browser history, unless cleared or removed.
You can 'selectively purge' individual videos from the YouTube History, either individually, or based on calendar date. If you watched only part of a video, the History link will start that video at the point where it was closed. The YouTube History affects the suggested videos displayed.
Just before George Michael died, he posted a 'deathbed' video on YouTube, which was removed hours later. I was writing an e-mail with a link to it while the video itself was playing in a different browser tab. By the time I sent the e-mail, the video had been removed, but I was able to prove that it had been on YouTube with a screen capture from my browser history.
Interesting. I just "cleared" the cache a couple of days ago but my geek friend said I wouldn't lose any data.
You don't lose data when you clear your browser's data cache. It can make previously viewed web pages load more slowly. The data cache was more important in the days when many people had slow dial-up Internet connections. It often took several minutes for certain pages to load. In Internet Explorer 5, the components of viewed web pages, (including images, site icons, and streaming video files), were stored in a subfolder of the hidden folder Application Data, named Temporary Internet Files. If you looked at the same web page again, it didn't take minutes for the page to load, because the consistent components of that page were loaded from the Temporary Internet Files folder, rather than from the website. Shortly after YouTube began, the viewed YouTube videos could be saved by retrieving them from the Temporary Internet Files folder; you couldn't open them in that folder, but you could cut or copy the file, and save it in a different folder not designated as temporary. Those YouTube videos were always .flv video file type.
Clearing the cookies cache for all or selected sites will result in the loss of some data.