Although the term "namespace URI" is widespread, the W3C Recommendation refers to it as the "namespace name". The specification is not entirely prescriptive about the precise rules for namespace names (it does not explicitly say that parsers must reject documents where the namespace name is not a valid Uniform Resource Identifier), and many XML parsers allow any character string to be used. In version 1.1 of the recommendation, the namespace name becomes an Internationalized Resource Identifier, which licenses the use of non-ASCII characters that in practice were already accepted by nearly all XML software. The term "namespace URI" persists, however, not only in popular usage, but also in many other specifications from W3C and elsewhere.