The update-ffmpeg and update-widevine scripts included in the Vivaldi install directory are provided to fix situations where proprietary media (AVC/H.264 and AAC) and Widevine (DRM/EME) respectively, are not setup correctly.
These scripts are primarily intended to be run as root (or under sudo) as they create and update files and directories that are root owned. However both support a command line option (--user) that adjusts their installation directories and thus allows them to be run without escalation.
The --user options were made for internal usage, with locally ‘unpacked’ copies of Vivaldi (i.e. not installed). However, it is possible to use them with standard installs (albeit with a little tweaking in the case of update-widevine).
To install proprietary media for just your current user issue the following:
/opt/vivaldi/update-ffmpeg --user
To install Widevine for just your current user, a couple more steps are required. This is because the script still attempts to adjust symlinks within the (typically root owned) Vivaldi install directory.
You can adjust the script as it runs, to disable the symlink creation, which would otherwise cause failure:
sed -r 's/^ *(rm|ln) -f.*/:/' /opt/vivaldi/update-widevine | sh -eus -- --user
Now create meta data in your Vivaldi user data directory that points to alternative Widevine install location–triple click to select the entire line:
echo "{\"Path\":\"$HOME/.local/lib/vivaldi/WidevineCdm\"}" > "${XDG_CONFIG_HOME:-$HOME/.config}/vivaldi/WidevineCdm/latest-component-updated-widevine-cdm"
Note: Change all “vivaldi” references in the above commands to “vivaldi-snapshot”, if you use the snapshot version.