emerge -pv =steam-base-9999

These are the packages that would be merged, in order:

Calculating dependencies... done!

!!! All ebuilds that could satisfy "=steam-base-9999" have been masked.
!!! One of the following masked packages is required to complete your request:
- games-util/steam-base-9999::steam-overlay (masked by: missing keyword)

For more information, see the MASKED PACKAGES section in the emerge
man page or refer to the Gentoo Handbook.