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Created February 17, 2021 23:41
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Example Cloudflare Worker script to add preloads to the `<head>` of a page and enable priority hints
// add this command to the CLI to enable priority hints
// set the site we are modifying
const site = '';
// do this on a fetch
addEventListener('fetch', event => {
const request = event.request
const url = new URL(request.url)
// the modified preloads to be added back into the head
var preloads = `
<link rel="preload" href="" as="style" importance="low">
<link rel="preload" href="" as="style" importance="low">
async function handleRequest(request) {
// store the URL
const url = new URL(request.url);
// disallow crawlers (write a robots.txt file)
if(url.pathname === "/robots.txt") {
return new Response('User-agent: *\nDisallow: /', {status: 200});
// when overrideHost is used in a WPT script, WPT sets x-host to original host i.e. site we want to proxy
// store the value of x-host
const xhost = request.headers.get('x-host');
// If this header is missing, abort
if(!xhost) {
return new Response('x-host header missing', {status: 403});
// set our hostname to that listed in the xhost header
url.hostname = xhost;
// look for header that allows us to bypass the transform entirely
const bypassTransform = request.headers.get('x-bypass-transform');
// get the accept header to allow us to examine the type of request it is
const acceptHeader = request.headers.get('accept');
// check our x-host header matches the site we are testing and we aren't trying to bypass the transformation
if(xhost === site && (!bypassTransform || (bypassTransform && bypassTransform.indexOf('true') === -1))){
// check to see the request is for the HTML page
if(acceptHeader && acceptHeader.indexOf('text/html') >= 0){
// store this particular request for modification
let oldResponse = await fetch(url.toString(), request)
// create a new response
let newResponse = new HTMLRewriter()
// IMPORTANT: add the preloads to the page
.on('head', new addPreloads())
// modify the HTML
// return the modified page
return newResponse
// otherwise just proxy the request unmodified
return fetch(url.toString(), request)
class addPreloads {
element(element) {
// prepend it to the specified element
element.prepend(preloads, {html: true});
// set any cookies (e.g. accept a cookie banner)
setCookie cookies_policy=true
// bypass the transforming the page HTML using a Worker?
addHeader x-bypass-transform:false
// this is the site we are modifying
// when the test encounters, overide with the worker URL
// go to this page and modify the html
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