WKWebView custom URL handler throws error when using css mask url: "Origin null is not allowed by Access-Control-Allow-Origin."

To avoid using file URLs, I'm using the WKURLSchemeHandler to create a custom url to load my assets into a webview.

This works great, except that certain ways of loading assets from the custom URL results in the error:

Failed to load resource: Origin null is not allowed by Access-Control-Allow-Origin.

Loading an asset in an image tag, css background property, or using a javascript import works. But using css mask, or javascript fetch does not.

You can see the problem by right-clicking in the window, and clicking show developer tools.

Css masks are used widely throughout the content in my webview, and I'm also using fetch. So I need both of these to work.

What is causing this error? Why does it only happen when using certain methods of loading resources?

Code Block swift
import SwiftUI
import WebKit
struct ContentView: View {
    var body: some View {
        HTMLView().frame(maxWidth: .infinity, maxHeight: .infinity)
    }
}
struct HTMLView: NSViewRepresentable {
    func makeNSView(context: Context) -> WKWebView {
        /* Create the custom url handler and webview */
        let config = WKWebViewConfiguration()
        config.setURLSchemeHandler(CustomURLHandler(), forURLScheme: "asset")
        let webview = WKWebView(frame: .zero, configuration: config)
        /* Enable developer tools so we can right click and show the inspector */
    	webview.configuration.preferences.setValue(true, forKey: "developerExtrasEnabled")
        /* Example html that does and doesn't work: */
        let html = """
<style>
#box {
    width: 100px;
    height: 100px;
    border: 1px solid green;
    /* This works: */
    background: url(asset://test.svg);
}
#box2 {
    width: 100px;
    height: 100px;
    border: 1px solid red;
    background: blue;
    /* This does not work: */
    -webkit-mask-image: url(asset://test.svg);
}
</style>
<p>This box loads the asset using the background property:</p>
<div id="box"></div>
<p>This box results in the error, and doesn't show the image with the css mask:</p>
<div id="box2"></div>
<script>
// Using javascript fetch also doesn't work:
fetch('asset://test.svg')
    .then(data => {
        console.log('Success:', data);
    })
    .catch((error) => {
        console.error('Error:', error);
    });
</script>
"""
        webview.loadHTMLString(html, baseURL: nil)
        return webview
    }
    func updateNSView(_ nsView: WKWebView, context: Context) {}
}
class CustomURLHandler: NSObject, WKURLSchemeHandler {
    func webView(_ webView: WKWebView, start urlSchemeTask: WKURLSchemeTask) {
        guard let url = urlSchemeTask.request.url else {
            return
        }
        /* Handle the custom url assets here. In this example I'm just returning a test svg: */
        let testImage = "<?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"no\"?><!DOCTYPE svg PUBLIC \"-//W3C//DTD SVG 1.1//EN\" \"http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd\"><svg width=\"100%\" height=\"100%\" viewBox=\"0 0 19 18\" version=\"1.1\" xmlns=\"http://www.w3.org/2000/svg\" xmlns:xlink=\"http://www.w3.org/1999/xlink\" xml:space=\"preserve\" xmlns:serif=\"http://www.serif.com/\" style=\"fill-rule:evenodd;clip-rule:evenodd;stroke-linejoin:round;stroke-miterlimit:2;\"><g transform=\"matrix(1,0,0,1,-540,-709.618)\"><g transform=\"matrix(0.5,0,0,0.666667,467,536)\"><g transform=\"matrix(2,0,0,1.5,-934,-804)\"><path d=\"M549,712.6C550.895,709 554.684,709 556.579,710.8C558.474,712.6 558.474,716.2 556.579,719.8C555.253,722.5 551.842,725.2 549,727C546.158,725.2 542.747,722.5 541.421,719.8C539.526,716.2 539.526,712.6 541.421,710.8C543.316,709 547.105,709 549,712.6Z\" style=\"fill:rgb(255,0,0);\"/></g></g></g></svg>"
        guard let data = testImage.data(using: .utf8) else { return }
        let urlResponse = URLResponse(url: url, mimeType: "image/svg+xml", expectedContentLength: data.count, textEncodingName: nil)
        urlSchemeTask.didReceive(urlResponse)
        urlSchemeTask.didReceive(data)
        urlSchemeTask.didFinish()
    }
    func webView(_ webView: WKWebView, stop urlSchemeTask: WKURLSchemeTask) {}
}