After a bit of a false start with an announcement made too early, Google has now announced Steam for Chrome OS and Chromebooks and now Valve has done their own brief announcement. So it's all officially official now.
From Valve's announcement:
As of Monday this week, an early version of Steam for Chrome OS has been made available to developers. Google and Valve have been collaborating on this project, which will ship to end users sometime in the future. Because Chrome OS is a Linux-based operating system, it can leverage much of the work that Valve has recently done for Steam Deck to allow games to run well even when they don't have a native Linux build. Google engineers have worked to greatly expand the capability of Chrome OS to make this possible.
Much like for Steam Deck, Valve has made it clear that developers don't actually need to test their own games, as it's "up to Google and Valve to make sure compatibility for Chrome OS is as robust as possible". That said, nothing stops developers ensuring their native Linux builds are in good shape or testing their Windows builds with Proton on a Linux desktop / Steam Deck. If developers do test, they can report issues directly to firstname.lastname@example.org.
Again, this is fantastic news for Linux gaming as a whole because Steam on Chrome OS is the Linux client. This gives developers more of a reason to care about Linux (on top of Steam Deck), and gives Valve another reason to keep the Linux client in good shape.