Steam On Chrome OS Finally Available In Alpha State For Select Chromebooks
The Chrome OS 14583.0.0 and later Dev channel includes an "early, alpha-quality" version of the Linux Steam client for Chrome OS.
As for the expected quality right now: "Alpha means anything can break. Due to the inherent instability of the Dev channel and the in-progress nature of this feature, we don’t recommend trying this on a Chromebook that you rely on for work, school, or other daily activities. You will encounter crashes, performance regressions, and never-before-seen bugs - that’s part of the fun!"
As for the initially supported Chromebooks capable of running Steam on Chrome OS:
Acer Chromebook 514 (CB514-1W)
Acer Chromebook 515 (CB515-1W)
Acer Chromebook Spin 713 (CP713-3W)
ASUS Chromebook Flip CX5 (CX5500)
ASUS Chromebook CX9 (CX9400)
HP Pro c640 G2 Chromebook
Lenovo 5i-14 Chromebook
Learn more about the current state of Steam on Chrome OS via Chromium.org.