Auto-Play tag currently shows the following description

Automatically running programs from inserted discs, USB flash drives, etc.

This suggests that something physical has to start. However, auto play could be for other reasons.

This question was tagged with auto-play. https://superuser.com/questions/1236061/on-a-mac-how-do-i-get-rid-of-disable-html5-autoplay-plugin

Obviously the trigger for auto-play is when the web page loads (or similar).

Due to the current description of the auto-play tag, I removed it from the post.

As I don't like editing the tags without verification first, can you please vote on one of my 2 answers below (or post your own).