According to HTML, when a web manifest is fetched, it should enforce the "JSON MIME type" type: https://html.spec.whatwg.org/#link-type-manifest Specifically: > To process this type of linked resource given a link element el, boolean success, response response, and byte sequence bodyBytes: > If response's Content-Type metadata is not a JSON MIME type, then set success to false. Where: A JSON MIME type is any MIME type whose subtype ends in "+json" or whose essence is "application/json" or "text/json".