There's a Chrome plugin that disables HTML5 video autoplay, with per-site toggles available. Most sites don't get that permission, but things like Youtube or other video sites (ahem) can get it easily.

[1] https://github.com/Eloston/disable-html5-autoplay/

Firefox has an option not to auto play movies but it can be hit and miss. It also doesn't cater to the middle line like youtube where I want auto play but not until the tab is focused.

Find the video content distribution provider's hosts, URLs, domains, and/or IPs, and firewall them.

This is about a 90% solution for me.

uMatrix

NoScript?