Based on RFC 2119: “Key words for use in RFCs to Indicate Requirement Levels”:
...
3. SHOULD -- This word, or the adjective "RECOMMENDED", mean that
there may exist valid reasons in particular circumstances to
ignore a particular item, but the full implications must be
understood and carefully weighed before choosing a different
course.
...
gem install rfc_spec
git clone git://github.com/seattlerb/rfc_spec