Update 2 (Aug 20): It's live again (opens in new tab), and it looks like it's staying up this time (opens in new tab).
Update: A Valve rep has confirmed that Steam.tv is real, it's in testing, and you weren't supposed to see it. "We are working on updating Steam Broadcasting for the Main Event of The International, Dota 2’s annual tournament," the rep said. "What people saw was a test feed that was inadvertently made public."
The International 2018 Main Event is scheduled to run August 20-25, so assuming that all goes reasonably well, we should be getting a proper look at Steam.tv soon.
Earlier today, a tweet (opens in new tab) from Pavel Djundik pointed us to the domain Steam.tv (opens in new tab), but at that time it was a blank site which simply read "Welcome to Steam.tv." A glance at the certificate showed it to be a legitimate Valve website, which was curious, but not quite enough to report. While we've suspected that Valve has Twitch dreams in its eyes, all we could do was speculate.
Later this evening, though, Valve apparently launched Steam.tv (opens in new tab) with a livestream of The International 2018 (opens in new tab) and chat bar. Cnet says (opens in new tab) that it's just the Dota 2 stream for now, and that you can log into your Steam account and access your friends list for group chats. It also has voice chat support in Chrome.
I'll have to take Cnet and Kotaku (opens in new tab)'s word for all that, because all I see is a blank page. I've tried coming at Steam.tv from a few locations via VPN just in case any region-targeting is going on, but still, nothing. Either the news spread fast enough to overwhelm the site, or Valve just took it down after a bit of fun.
Whatever the case, this seems to be a very limited test of a website that expands on Steam's streaming functionality, creating a more Twitch-like experience for viewers. We don't know how far Valve plans to take it, but it's a safe bet that it won't be limited to Dota 2 streams.
We'll let you know if Valve says anything about its Friday night tinkering.