Both json standard (ECMA-404) [1] and the new json rfc 7159 [2] do not mention
the encoding detection.

[1] http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-404.pdf
[2] https://tools.ietf.org/html/rfc7159#section-8.1

From the rfc:

> JSON text SHALL be encoded in UTF-8, UTF-16, or UTF-32.  The default
  encoding is UTF-8, and JSON texts that are encoded in UTF-8 are
  interoperable in the sense that they will be read successfully by the
  maximum number of implementations; there are many implementations
  that cannot successfully read texts in other encodings (such as
  UTF-16 and UTF-32).

  Implementations MUST NOT add a byte order mark to the beginning of a
  JSON text.  In the interests of interoperability, implementations
  that parse JSON texts MAY ignore the presence of a byte order mark
  rather than treating it as an error.