Here I google.com it for you.
http://www.pathname.com/fhs/pub/fhs-2.3.html#THEUSRHIERARCHY
EDIT: Why is it okay to NOT look up a Standard and be uncertain in your comments when it can be found in 20 seconds tops on the Internet?
'common unix standard' is whatever POSIX states as standard.
I gave this down voted "I googled it for you" because people need to just look up what the Standard is and than speak from a position of knowledge and not some vague non-answer.
For the still lazy the Unix File Hierarchy Standard for /usr
/usr is the second major section of the filesystem. /usr is shareable, read-only data. That means that /usr should be shareable between various FHS-compliant hosts and must not be written to. Any information that is host-specific or varies with time is stored elsewhere.
Large software packages must not use a direct subdirectory under the /usr hierarchy.
> The /usr/local hierarchy is for use by the system administrator when installing software locally. It needs to be safe from being overwritten when the system software is updated. It may be used for programs and data that are shareable amongst a group of hosts, but not found in /usr.
> Locally installed software must be placed within /usr/local rather than /usr unless it is being installed to replace or upgrade software in /usr.
http://www.pathname.com/fhs/pub/fhs-2.3.html#USRLOCALLOCALHI...
It says "The /usr/local hierarchy is for use by the system administrator when installing software locally."
That seems to kind of conflict with what you quoted "Large software packages must not use a direct subdirectory under the /usr hierarchy.", unless they mean a vendor-specific directory rather than "/usr/local".
I dunno, all I can say is that in my experience on various unixes, it is a "common unix standard" to install software that is not controlled by the OS package manager to /usr/local. I don't interpret "common unix standard" as "FHS" though -- if you mean FHS, say FHS. To me "common unix standard" just means "something that is commonly done on unix". And installing locally-installed (not using OS distro) software to /usr/local is pretty commonly done, as many people can attest. That's why homebrew chose it, they didn't just make it up.
And no, I am not the one downvoting you.