mobile-spec failure in XMLHttpRequest.spec.4 is a known WKWebView issue, where CORS is now respected and cannot load files from file:// urls, only http*:// urls are allowed.
With CSP off to rule things out:
XHR to yourself of course works, but doesn't really make sense for
real-world use. XHR to a sibling file, parent file, or any child file
results in the error ""Cross origin requests are only supported for
index.html is the currently loaded file in the WebView. From it, you
can't load parent.xml, sibling.xml nor child.xml using XHR according
to my tests.
The workaround is implementing