UTF-16

UTF-16 (16-bit Unicode Transformation Format) is a character encoding that supports all 1,112,064 valid code points of Unicode. 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, including most emoji and important CJK characters such as for personal and place names. UTF-16 is used by the Windows API, and by many programming environments such as Java and Qt. The variable-length character of UTF-16, combined with the fact that most characters are not variable-length (so variable length is rarely tested), has led to many bugs in software, including in Windows itself. UTF-16 is the only encoding (still) allowed on the web that is incompatible with 8-bit ASCII. It has never gained popularity on the web, where it is declared by under 0.003% of public web pages (and even then, the web pages are most likely also using UTF-8). UTF-8, by comparison, gained dominance years ago and accounted for 99.1% of all web pages by 2026. 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.