Note that Firefox' autoplay blocker breaks some video sites because the Javascript assumes the video is playing when it really isn't. However, with Firefox, videos never autoplay. Chrome's autoplay does not always work, but it doesn't seem to break sites.

I can't seem to find the issue, but in the Safari issue tracker, the point was made that given the way HTML5 video and APIs work, it is impossible to reliably prevent autoplay without breaking some sites. It seems that Firefox went for "reliably prevent autoplay" while Chrome went for "without breaking sites".

FWIW, I switched from Chrome to Firefox because of the autoplay issue.