I don’t think it will be a significant problem.
It’s a rare site that would intentionally have its embeded videos autoplay. It’s possible this is a bug in getyarn.io’s embed code. Or possibly some attempt to get “views.”
So we might want to contact them and tell them the problem. And then tell Discourse if they don’t fix it, as they could possibly change how they handle their links. And, if that fails, maybe we blacklist the site from getting a card–assuming that’s possible.
In the mean time, as a work around, you may also want to take advantage of a feature that has been part of Chrome for a while, and I believe is part of Firefox, too. You can “Mute site,” to avoid any audio from that site, by right clicking on the tab.
Though a side effect will be that you may have to open any videos you see posted in a separate tab to get audio, or temporarily unmute the site for that.