> Maybe because they don't like people downloading the videos they host?
Sites are not protected when they use proprietary technology either. It is
possible to sniff the network traffic with tools like tcpdump and wireshark
when the proprietary Adobe flash plugin is running, and then extract the clip
from the dump or download it from extracted link in the dump. It is an
imaginary protection.
> For HTML5 video to work on YouTube, I had to uninstall...
Linterna Mágica tries to respect the switch to HTML5 since version 0.0.10.
All HTML5 players that are detected in websites have higher priority and are
not treated and replaced as with flash players. Maybe other projects could
also try this approach. File bug reports. I don't see how this could work for
Gnash though.