Three different cases are handled:
1. Most video pages use the Video.js framework. I extract the arguments
of the JavaScript `player.updateSrc()` call to get the video details
(one or two different formats)
2. Some video pages include an iframe which embeds a video from another
site, such as YouTube, XHamster, XVideos and possibly more.
3. Also supports `<img>` tag for gifs or plain simple pictures.
This does not yet extract any metadata besides the essentials.
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