the recent "steam broadcasting" (live video game streams served via steam's service)

uses DASH and is works with pure HTML5 (no flash needed)

In order for DASH to work you need Media Source Extensions support. Firefox for example doesn't support them yet.

They're hidden behind an about:config flag - media.mediasource.enabled

The latest version of developer edition/aurora just enabled them by default, so version 36. They may be only ~12 weeks away from becoming default in the stable build.

Thanks, I didn't know that setting already made it into the stable build. Now I can watch Youtube in high resolution (and in VP9 with that!) without using Flash. Why is it still disabled though, there are some bugs left?

UPDATE: some remaining issues are listed here: https://bugzilla.mozilla.org/show_bug.cgi?id=778617

Not in safari or iOS

According to Steam it works in the Steam Client, Google Chrome, and Safari: https://support.steampowered.com/kb_article.php?ref=6730-TOA...

If they just say 'Safari', it probably doesn't work on iOS at the moment. But it should work on your Mac unless an extension/addon is interfering.

Safari on Mac now supports MSE, on which there are multiple DASH implementations. Safari on iOS is behind, but it's not hard to guess where it's going.