After installing `chromium-codecs-ffmpeg` (as indicated in an answer under the linked question) this was not fixed; but installing`chromium-codecs-ffmpeg-extra` instead (installing one package removes the other) for the purpose of playing Vimeo videos (as indicated here) solved for Chromium all the problems I had with both Opera and Chromium (not playing live streams and a lot of other non-live online video).
(N.B The problems were solved only for Chromium, not for Opera.)