YouTube videos don’t auto-play, so I don’t think Twitch embeds should either. Also, the display is small compared to YouTube (300x150 vs. 480x270). A consistent width for embeds would be cool.
Oh and the preview display for Twitch channels is kind of odd (it shows the channel’s profile image but only the embed is shown once the post is published).
Unfortunately this is what Twitch proves in their meta data that Discourse uses to embed.
It would require creating a custom Twitch Onebox parser which would do it’s own logic. That or similar to LazyYT, create a LazyTwitch which would make a better preview and click to play without actually loading the Twitch embed.
This is a Twitch issue as @chapel noted. Maybe they have improved it by now?
Just found this topic doing a random search for something unrelated. If this is still a problem for you, I developed a Twitch Onebox to address this.
https://meta.discourse.org/t/twitch-onebox-for-discourse/54997
I don’t think Twitch oneboxes autoplay any more do they @eviltrout?
We should probably handle Twitch natively with a custom player onebox as they are big enough to justify it.
I added better twitch embedding support in the onebox gem:
https://github.com/discourse/onebox/commit/7f4b9ceacee10a3b5a580393191c606e2cd44594
Let’s test with this brazilian gem 
