alt Hacker NewsAdditional Detail: it is specifically utf-16 little endian when a byte order mark is not used, which is the opposite of the recommended choice of big endian in the RFC.
Worse are the byte order marks required to support both endians that end up in files.
The development of Windows NT based on UCS-2 precedes the RFC by roughly a decade, and little-endian was the natural choice for the Intel PC platform. Obviously the endianness had to remain the same when UCS-2 was extended to support UTF-16.