@namespace foo url('http://www.mysite.org/2024/foodoc')

Note that namespace names are just strings, so the fact that the SVG namespace name also looks like a URI isn't important. URIs are commonly used because they are unique, but the intention is not to "link" somewhere. (In fact, URIs are used so frequently that the term "namespace URI" is commonly used instead of "namespace name".)

even if you use the  syntax, it is treated as a literal . So the URI does not necessarily need to satisfy the requirements of the  data type. All strings — including the empty string and strings representing invalid URIs — are valid namespace names when using the @namespace at-rule.

https://nimbupani.com/spacing-out-on-css-namespaces.html says:

"HTML5 allows inline SVG and MathML, which means you can style HTML,
inline SVG and MathML elements using the same stylesheet."

It implies that there isn't way for users to create a new namespace in a browser.