For the html5 help page.
As shown on the second video, the uses is prompted to run some command that downloads the relevant chromium ffmpeg codecs from ubuntu's ppa, extract libffmpeg from it and place it in ~/.local/lib/vivaldi. Neat solution, but it won't work on opera because opera is not coded to look in that folder.
It is also not system wide, which means that it has to be run for every dingle user on the system.
Moreover, for anyone who is not on ubuntu, this simply installs a foreign (= built with a different set of compilers and libraries) library on the system. And, for me, one more reason to not install vivaldi, ever,
Also, this will stop working soon, now that ubuntu provides chromium and its "family" of packages only as snaps.
That is the reason that vivaldi does not seem to depend on any ffmpeg lib and neither carry a libffmpeg on its own. As it seems, vivaldi is built the same way opera and ubuntu's chromium are built, without support for ffmpeg's libraries
For the widevine help page
The decribed procedure runs a vivaldi-specific script, this one
And all it does is to download libwidevinecdm from google, which seems to be in a single zip instead of being bundled with chrome like it used, and place it in the right folder, system-wide or per-user.
I think this can be applied to opera, provided that it gets the relevant changes so as to place the lib in the right place.
For the flash help page
Seriously now, the instructions on the ubuntu part of the page are simply perfect. That is the best way to install flash on an ubuntu-based distro, from the partner repo.
However this part
We look in the install location used by the Official Flash package provided in the Canonical partner repositories on Ubuntu Linux.
and this part
You can also use this script if your distro does not provide a Pepper Flash package.
are completely wrong, because both imply the usage of ubuntu's package on other distros. I am pretty sure every distro has ppapi flash as a package on its repo.
PSA. The above DO NOT APPLY to the snap version of opera.
Feel free to ask anything