Only for existing customers who use this sign-in. New accounts can not be created in this way.
Mozilla/5.0 (Macintosh; U; MacOS X 10_8_5; en-US; Valve Steam Client/default/1490228413; ) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/49.0.2623.110 Safari/537.36
Valve/Steam HTTP Client 1.0