У вашего броузера проблема в совместимости с HTML5
Link to files:
http://store.steampowered.com/steamos/buildyourown
Command:
~/post_logon.sh
Note this guide uses the "Custom Installation" which will work on hard drives smaller than 1 TB
Update 1: A common issue I ran into was at 4:49 where in place of the 'Steam' icon was a 'Return to Steam' icon instead. Try launching Terminal and typing "steam" and pressing enter. Or trial and error seems to be another way to resolve it, rebooting, choosing gnome, xsession, steamos, etc.
Update 2: Valve released an ISO that should help with non-UEFI PCs and dual booting concerns. Note I have not tested this myself so you're on your own.
http://steamcommunity.com/groups/steamuniverse/discussions/1/648817378243644036/