Function has_external_ref 

fn has_external_ref(lower: &str) -> bool

true if a fetching attribute/function (href, src, CSS url(...), @import) points at a remote scheme (http, https, or protocol-relative //). Deliberately does not match xmlns="http://…" namespace URLs (they carry no href/src/url(), nor self-contained data: URIs. Coarse by design — see validate_svg for why that is sufficient in the <img> render context.