Hello @sylware, this is a repeat of #7059, which points back to https://github.com/ValveSoftware/steam-runtime/issues/253#issuecomment-619064429.
It's unlikely that anything has changed since you encountered this 3 months ago, but please copy your system information from Steam (Steam -> Help -> System Information) and put it in a gist, then include a link to the gist in this issue report.
the steam-runtime is almost able to run google libcef, only the dbus stuff is missing.
in the log, libcef is complaining about a missing symbol FcConfigGetSysRoot