I suppose my netbook might be missing a video codec, but the videos play ok in Chrome so I don't think it would be a missing codec.
Chrome doesn't rely on the OS to play videos, it uses its own library afaik.
What does opera://media-internals/ says when you try to load the video?
Wow, I didn't know about opera://media-internals, thanks!
Under the Player tab it says:
url: https://assets.stuff.co.nz/video/production/1451429022625-darts best.mp4
error: FFmpegDemuxer: open context failed
pipeline_error: demuxer: could not open
Does that tell you anything?