Cloudflare Worker code used when modifying the priority hints for the Nike UK homepage.

workre.js

// set the site we are modifying
const site = 'www.nike.com';

// do this on a fetch
addEventListener('fetch', event => {
  const request = event.request
  const url = new URL(request.url)
  event.respondWith(handleRequest(request)) 
});

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()
        // drop the preload importance
        .on('link[href="https://www.nike.com/assets/experience/ciclp/landing-pages/static/v2/common/vendor.6a1e0df25e0509359e59.js"]', new SetPriorityLow())
        .on('link[href="https://www.nike.com/assets/experience/ciclp/landing-pages/static/v2/1494-4685d103b4e/client.7b2ff9cd8475a0f2cf37.js"]', new SetPriorityLow())
        // modify the HTML
        .transform(oldResponse)
        // 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});
  }
}

class removePreloads {
  element(element) {
    element.remove();
  }
}

class SetPriorityLow {
  element(e) {
     e.setAttribute("importance", "low");
  }
}

class SetPriorityHigh {
  element(e) {
     e.setAttribute("importance", "high");
  }
}

class SetPriorityAuto {
  element(e) {
     e.setAttribute("importance", "auto");
  }
}