The new stable Steam Client for Windows, Linux, and Mac brings an all-new chat feature that is also available for web browsers, GPU acceleration in web views by default, support for the PlayStation 3 controller, 15 Xbox controllers, and more. In the near future, Windows XP and Vista will no longer be supported by the Steam Client.
https://www.notebookcheck.net/Steam-gets-an-all-new-chat-system-for-both-the-Steam-Client-and-web-browsers.318208.0.html