3. For example, I'm using Debian wtih Gnash (
http://en.wikipedia.org/wiki/Gnash) but not Adobe Flash, and I can watch Youtube videos. But not videos on other sites. Apparently most flash video players use some feature which is specific to Adobe Flash and are not compatible with Gnash. However, it is only a problem with player (i.e. a thing which shows buttons and whatnot), not a problem with video format itself.
4. FWIW Gnash uses ffmpeg/avcodec.
EDIT: Oh, it isn't video, it is Flash animation. I've downloaded it and it seems to be work fine with Gnash, so it is open source enough, I guess.
Okay so maybe the problem is the browsers, they claim a plugin is needed to view flash, and they aren't offering any free open source flash viewing tool they just send me to macromedia or adobe or whoever and they say sorry we are not going to be supporting Linux.
So maybe the free open source browser folks just don't know yet that there is a free open source flash viewer they can point people to, or they have not plugged it in yet because so far the commerical/propriery one has not actually stopped support yet they are just saying they are going to.
For youtube I often used to be told by the browser it simply cannot find any plugin at all that can view whatever it was that it thought i needed a plugin to view. I haven't actually run into that lately for some reason but maybe mostly because I got used to not bothering to click on links to youtube.
-MarkM-