pmeenan

Priority Hints is rolling out to Chrome in the 101 release which is currently available in the Dev/Beta channel of Chrome and available in WebPageTest when using the Chrome Canary browser selection.

To make it easier to experiment with priority hints (particularly for LCP images) without making production changes, I set up a couple of public Cloudflare Workers that can be used dynamically with WebPageTest to inject priority hints into existing pages and to preload arbitrary images when combined with WebPageTest's overrideHost script command.

Injecting Priority Hints

There is a cloudflare worker at hint.perf.workers.dev that will take a CSS selector from the x-hint HTTP header and add fetchpriority=high to any elements in the HTML that match the selector. The easiest way to experiment with this is to use Chrome's dev tools locally, identify the element that hosts the imag

pmeenan

addEventListener('fetch', event => {
 event.respondWith(handleRequest(event.request))
});

async function handleRequest(request) {
  const url = new URL(request.url);
  const host = request.headers.get('x-host');
  if(!host) {
    return new Response('x-host header missing', {status: 403});
  }

pmeenan

[RUMSpeedIndex]
var RUMSpeedIndex = function(win) {
  win = win || window;
  var doc = win.document;
    
  /****************************************************************************
    Support Routines
  ****************************************************************************/
  // Get the rect for the visible portion of the provided DOM element
  var GetElementViewportRect = function(el) {

pmeenan

// Support routines for automatically reporting user timing for common analytics platforms
// Currently supports Google Analytics, Boomerang and SOASTA mPulse
// In the case of boomerang, you will need to map the event names you want reported
// to timer names (for mPulse these need to be custom0, custom1, etc) using a global variable:
// rumMapping = {'aft': 'custom0'};
(function() {
var wtt = function(n, t, b) {
  t = Math.round(t);
  if (t >= 0 && t < 3600000) {
    // Google Analytics