UTF-16 (16-bit Unicode Transformation Format) is a character encoding capable of encoding all 1,112,064 valid code points of Unicode (in fact this number of code points is dictated by the design of UTF-16). The encoding is variable-length, as code points are encoded with one or two 16-bit code units. UTF-16 arose from an earlier obsolete fixed-width 16-bit encoding, now known as UCS-2 (for 2-byte Universal Character Set), once it became clear that more than 216 (65,536) code points were needed.
|Classification||Unicode Transformation Format, variable-width encoding|
|Transforms / Encodes||ISO/IEC 10646 (Unicode)|
UTF-16 is the only web-encoding incompatible with ASCII and never gained popularity on the web, where it is declared by under 0.002% (little over 1 thousandth of 1 percent) of web pages (and many of these are actually UTF-8 because of "contradictory character encoding specifications" and/or "incorrect character encoding defined"). UTF-8, by comparison, accounts for 98% of all web pages. The Web Hypertext Application Technology Working Group (WHATWG) considers UTF-8 "the mandatory encoding for all [text]" and that for security reasons browser applications should not use UTF-16.