https://bugs.winehq.org/show_bug.cgi?id=39403
--- Comment #41 from lehmannwer@gmail.com --- Yes, seems like CEF is the problem. I looked here https://en.wikipedia.org/wiki/Chromium_Embedded_Framework
which confirms that Steam uses that. As also Spotify is listed, I tried to install it and it did not work. I had used Spotify in wine more that two years ago and it worked fine.
Conclusion: wine needs to implement that CEF thing to solve this steam client problem and probably many others.