Linux users will now be able to play more Windows games on Steam thanks to a new beta version of Steam Play released yesterday.
For the new beta, Valve has helped design and fund a new modified distribution of Wine, an existing open-source compatibility project. The modified version is called Proton, and it will mean that Windows games with no current Linux version can be installed and played through the Linux Steam client, "complete with native Steamworks and OpenVR support".
In Proton, DirectX 11 and 12 implementations are based on the Vulkan API, which Valve said should improve compatibility and performance. Fullscreen support has been improved, as has controller support: games will now recognise all controllers supported by Steam. Performance for multi-threaded games has also been "greatly improved compared to vanilla Wine", Valve said in a Steam post.
The beta will only initially support 27 games, including Into The Breach, Doom and STALKER: Shadow of Chernobyl, but more games will be added "in the near future". You can also use an override switch on the Linux Steam client to play non-whitelisted games (head to Steam Play options), but Valve only recommends that for "enthusiast users". It might not work for everything, though: games with "complex DRM or anti-cheat systems" may never run on Steam Play, Valve said.
The 27 supported games will "will not be offered for purchase or marked as supported on Linux" during the initial beta, which suggests that you already have to own those games on Steam to take part (it could be that they're free on the Linux Steam client for a limited time, but I doubt it—I've reached out to Valve to clarify).
You can find out more about the beta, the full list of supported games, and how Proton works in the Steam post.
And if you're curious, here's a guide to playing Windows games in Linux.