Modifying such headers is forbidden because the user agent retains full control over them. Names starting with `
Sec-` are reserved for creating new headers safe from APIs using Fetch that grant developers control over headers, such as
Forbidden header names start with
Sec-, or are one of the following names:
User-Agent header is no longer forbidden, as per spec — see forbidden header name list (this was implemented in Firefox 43) — it can now be set in a Fetch Headers object, or via XHR setRequestHeader(). However, Chrome will silently drop the header from Fetch requests (see Chromium bug 571722).