Safari responds to a successful "fetch" of a file:// URL with a status of 0

I built a web application that is designed to be run from my application bundle by using webView.loadFileURL(htmlFile, allowingReadAccessTo: projectFolder) on a WKWebView.

Inside of the site's folder is a directory of svg images (from the ionicons project). At runtime the code tries to load one of those images using:

        req = fetch(url).then((rsp) => {
          if (rsp.ok) {
            return rsp.text().then((svgContent) => {
              if (svgContent && sanitize !== false) {
                svgContent = validateContent(svgContent);
              }

(code from the ionicons project).

The rsp.ok check simply examines the response to the fetch request to see if the status code is in the range 200-299. For the "file://" URL, however Safari on both MacOS and iOS returns 0 in the status.

In spite of that 0, however the request has succeeded and the browser has the content of the file. If I modify the code to ignore rsp.ok the following call accessing rsp.text() succeeds.

I'm not sure what the expectations are for a file:// URL passed through fetch. Is the 0 status code a bug in Webkit or Safari, or is it an expected result for a file:// URL and the code making the fetch request needs work?