Comment # 27 on bug 929471 from Dr. Werner Fink

Rational for choosing a hostname without domain name can be found in RFC 1178
and RFC 1123

  https://tools.ietf.org/html/rfc1178

[...]
      Avoid domain names.

         For technical reasons, domain names should be avoided.  In
         particular, name resolution of non-absolute hostnames is
         problematic.  Resolvers will check names against domains before
         checking them against hostnames.  But we have seen instances of
         mailers that refuse to treat single token names as domains.
         For example, assume that you mail to "libes@rutgers" from
         yale.edu.  Depending upon the implementation, the mail may go
         to rutgers.edu or rutgers.yale.edu (assuming both exist).
[...]

  https://tools.ietf.org/html/rfc1123

   2.1  Host Names and Numbers

      The syntax of a legal Internet host name was specified in RFC-952
      [DNS:4].  One aspect of host name syntax is hereby changed: the
      restriction on the first character is relaxed to allow either a
      letter or a digit.  Host software MUST support this more liberal
      syntax.

      Host software MUST handle host names of up to 63 characters and
      SHOULD handle host names of up to 255 characters.
[...]

Btw: the array for the hostname in /proc/sys/kernel/hostname has 64 characters
which includes the trailing ASCII null as well as the hostname.