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.