Videos do not play on Linux

Thought developers had solved this long ago.

Vivaldi 5.3.2679.68
Gentoo
ffmpeg 4.4.2 (built with chromium use flag): “Builds libffmpeg.so to enable media playback in Chromium-based browsers like Opera and Vivaldi.”

Went to:

help.vivaldi.com/desktop/media/html5-proprietary-media-on-linux/

Restarted Vivaldi several times. Video never plays with no indication of the problem.

Every time I start Vivaldi (from a terminal), i get:

“Proprietary media’ support is not installed. Attempting to fix this for the next restart.”

I also see a popup that says:

“Media support

Vivaldi lacks a suitable component to play HTML5 ‘proprietary’ media (H.264/AAC)”

Vivaldi gives no other indication of why it can’t play video.