Steam doesn't use bare WebKit but Chromium, presuming they stay up to date (my install registers as Chromium 18, so doubtful) and don't switch, they'll be running Blink instead.

https://developer.valvesoftware.com/wiki/Chromium_Embedded_F...

(Unrelated: their embedded Chromium seems to be hard-coded to Windows-like shortcuts; even in the address bar)