The W3C has all their specs, as far as I know, use Uniform Resource Identifiers (URIs), and most use the subset of Uniform Resource Locators (URLs). There seems

The W3C has all their specs, as far as I know, use Uniform Resource Identifiers (URIs), and most use the subset of Uniform Resource Locators (URLs). There seems to be a strong sentiment against using other types of identifiers, based on my experience on the Extensible Resource Identifier (XRI) technical committee in OASIS. So the chance that they'd create a new one for XPath was probably very unlikely, in my opinion. Even non-URL URIs might have been better for XPATH namespaces. As an aside, I haven't used XPath in a few years, but use JSONPath / JSONPointer on multiple days each week, and highly recommend them. I find XML to be great for many things, but I find JSON much easier to work with.