in bindings/xlb/src/edge_binding.ts [302:310]
async fetch_challenge_page(options: EdgeRequestInit): Promise<EdgeResponse> {
const resp = await this.fetch(
new FetchApiRequest(new Request(this.config.challengePageUrl || CHALLENGE_PAGE_URL, options)),
);
const text = await resp.text();
const newResp = new ImmediateResponse(text, { status: resp.status });
resp.getHeaders().forEach((v, k) => newResp.addHeader(k, v));
return newResp;
}