You will need to install a user script manager extension to install this script.
After YouTube switched to using HTML5 by default, I wondered why Mark Pilgrim's classic "Disable video autoplay" UserScript from Dive into HTML5 wasn't working, and why it didn't keep Vine videos from autoplaying; I didn't notice anything wrong with the script itself, and I learned that YouTube uses a different method to make videos autoplay when I saw that the
autoplay attribute was already
false, but I was still inspired to extend the old script to turn off autoplay for HTML5 audio. I might figure out how to make this keep Youtube and Vine from autoplaying too.
I added a workaround for YouTube (and possibly Vine) based on this: https://greasyfork.org/en/scripts/6487-pause-all-html5-videos-on-load
I have also incorporated more sophisticated anti-autoplay script based on Stop Youtube HTML5 Autoplay by Leslie P. Polzer of PORT ZERO firstname.lastname@example.org: http://www.port-zero.com/en/chrome-plugin-stop-html5-autoplay/
I added a workaround for the older Flash-based YouTube, based on this: http://userscripts-mirror.org/scripts/review/100858
I also added an attempted workaround for some videos on Tumblr, based on this: https://greasyfork.org/en/scripts/921-tumblr-disable-autoplay
For the benefit of those who would use this script in IE7Pro, I have used Eduardo Cereto Carvalho's cross-browser implementation of addEventListener: https://gist.github.com/eduardocereto/955642