Per the fetch spec, if a Headers object has the "request-no-cors" guard, appending or setting a header will only succeed if the header is a no-CORS-safelisted request header (https://fetch.spec.whatwg.org/#no-cors-safelisted-request-header), which only includes the `Accept`, `Accept-Language`, `Content-Language` and `Content-Type` header names, and doesn't include all header values. However, Webkit seems to accept any header as long as its value is the empty string. A test for this is running the following code on the console: const request = new Request("https://example.com", {mode: "no-cors"}); request.headers.append("X-Test", "fsdfsd"); request.headers.has("X-Test"); // false request.headers.append("X-Test", ""); request.headers.has("X-Test"); // true The last line prints false in Firefox and Chromium.