This indicates that indexing on a DomString occurs in units of 16 bits. This must not be misunderstood to mean that a DomString can store arbitrary 16-bit units. A DomString is a character string encoded in UTF-16; this means that the restrictions of UTF-16 as well as the other relevant restrictions on character strings must be maintained. A single character, for example in the form of a numeric character reference, may correspond to one or two 16-bit units.
1.5.2