Our favourite conference of the year is back: CSS Day ✨ CSS legends, browser engineers, and web platform experts sharing talks on modern CSS, accessibility, design systems, browser internals, and much more. cssday.nl See you next week in Amsterdam? ☕️

I did a 10-minute DevTools audit of a top 10k site with real RUM tooling installed. Every Core Web Vitals metric: poor. The hero image isn't preloaded. The tag manager, consent tool, and coupon service are. Organizational influence showed up in the preloads. www.ethangardner.com/posts/lightn...

I'm really excited for this one, especially for design systems where it will shed more light on component performance.

In response to our objection to the Prompt API, some folks replied "well, fix it. Propose something better". This is something we end up doing a lot at Firefox…

It's almost time to implement the Soft Navigations API to accurately measure Core Web Vitals for SPAs (React, Vue, Angular, etc.). We applied your feedback to build the expected final shape. Conduct a final test during the origin trial in Chrome 147-149 → goo.gle/4ttf9eP

Layout shifts often only happen at specific screen sizes or after a user interaction. That's why I built a new free tool: the CLS checker tests your site at 5 different viewport sizes and scrolls down the page. Try it out here: www.debugbear.com/test/cls

I'm very excited about this! Have seen some really significant impact in our tests. With the sizes of websites and JS payloads continuing to increase, this is both important and impactful.

I recently analyzed data from the HTTP Archive to see how the web has changed since the launch and adoption of AI tools. The results show a clear trend. The web is getting heavier at an accelerated rate. www.ethangardner.com/posts/object...

Performance isn’t just about the first click; it’s about the entire journey. By making sure your pages are eligible for bfcache, you remove the technical hurdles of the network and CPU, creating a browsing experience that feels as fast as a native app. www.ethangardner.com/posts/bfcach...

I'm listening to an audio book and the author said that he encountered software with more bugs than the Amazon, and I am totally stealing that expression.

🎨 I gotta say, my latest is a little treasure trove: what even is `contain`?! csswizardry.com/2026/04/what...

⚡WebPerformance Report is more than a report in your inbox. Users also get access to their Board to manage reports, review deliveries, and follow Performance, Accessibility, and Security trends over time. Get your report here: 👉https://webperformancereport.com/ #webperf

p75 and p90 JavaScript bytes on the web have exploded since May, 2025—easily the largest 10 month period based on HTTP Archive data since 2023. The long-tail is getting worse, very quickly. Gemini, what happened in May 2025?

⚡Featured posts this week: ✨ Telescope Test Agent: Mike Kozicki @nywebperf.com ✨ Agentic SEO for e-commerce: Emilie Wilhelm ✨ The Hidden Costs Behind Modern Frontend: Juntao Qiu ✨ Web Performance Archetypes: @ethangardner.com rdner 🙏 Special thanks to the authors! #webperf

✨ Core Web Vitals Summit: The Videos ✨ I'll be sharing the recorded sessions from the Summit stream, and some brief commentary on some takeaways and significance. All are welcome to join the conversation! 📆 Monday March 23rd ⏰ 1pm EST, 10am PST 🔗 www.youtube.com/live/LXYcknO...

Is it really possible to anticipate shifts in RUM data from immediate LAB results, using machine learning (ML)? We're going to find out with @ethangardner.com. Joins us for that conversation ✨ SPDY STREAM 027 ✨ 📆 Tomorrow, Wednesday March 18th ⏰ 1pm EST, 10am PST 🔗 www.youtube.com/live/ZEy0lRq...

Your website has a UX Score and you've probably never seen it. It's based on real Chrome user data from real visits. Not a lab test. Not a guess. Enter any URL, get your score in 10 seconds: www.iron-out.io/benchmark Free. No login 🚀

Yes, I know it's Friday… but I deployed this anyway! 😎 We added the No-Vary-Search response header to a bunch of sites to ignore tracking parameters like `utm_*`, `gclid`, and `fbclid`. They’re consumed by third party tracking scripts, but they should not create .. 1/4

A slow UX affects people’s perception of three important brand elements that are completely unrelated to page speed: content, design, and usability. Here's why that takeaway is relevant in an increasingly competition online landscape 👉 www.speedcurve.com/blog/brand-p... #webperf #ux #pagespeed

Web performance metrics aren’t just engineering diagnostics. They’re large-scale UX research. They reveal friction, bias, and experience gaps usability studies alone can’t see. I wrote about this intersection: www.ethangardner.com/posts/web-pe... #ux #webperf

How I use Web Components with ShadowDOM in a Design System. www.ethangardner.com/posts/shadow...

Back to work tomorrow. This is my reflection on 2025. www.ethangardner.com/posts/2025-y...

Web performance calendar day 3 when @ethangardner.com shows us how to train a ML model on synthetic perf metrics and use it to predict approximate field data before a page/feature goes live. calendar.perfplanet.com/2025/traffic...

How's the mobile CPU landscape evolving? Thanks to process node improvements, a mid-range Android from 2025 is (finally!) as fast as 2017's iPhone 8, but a low-end (~$100) smartphone isn't yet as speedy as a decade-old iPhone 6s.

My talk at Performance.sync() pre-conference meetup organized by @mozilla.org in Amsterdam last week: www.youtube.com/watch?v=2cv6...

If you are looking for the slides from my PerfNow 2025 talk, Web Performance Allies, they are at speakerdeck.com/ethangardner...

Just like web performance work, accessibility winds up being less expensive the earlier you implement it. Build accessibility in from the start, rather than bolting it on - make it part of the process. @marcysutton.com #webperf #PerfNow #a11y

performance.now() is coming! @ethangardner.com will offer practical tips on presenting engineering data as a competitive advantage, a revenue driver, and a tool for enhancing user experience to help make performance optimization a strategic priority. perfnow.nl/speakers.htm...

performance.now() is coming! The night before the conference, 29th of October, @mozilla.org organises performance.sync(), the pre-meetup where you can meet your peers to mentally prepare for two days of information overload. They're also looking for a few people to present. luma.com/ziykhh4y

Come see me in Amsterdam! This lineup is loaded with great speakers in the #webperf community, and it's going to be a good time.

The videos from our latest episode are up! Come catch a pair of great talks by @verlok.bsky.social and @ethangardner.com! www.youtube.com/playlist?lis... #webperf

Hey folks, the next episode of PerformanceObserver is now scheduled for 16 April with @verlok.bsky.social and @ethangardner.com ! See all details at performanceobserver.dev including add to calendar links with the join instructions. #webperf

What is everyone using for revenue reporting and invoicing for consulting and freelance work these days?

If this isn't a testament to nature, I don't know what is.

For the last year, I've been ramping up on AI. If I was to start my learning journey today, these are some of the resources I'd use. www.ethangardner.com/posts/ai-onr...

Good things are happening with URLPattern. I've been waiting for it to land in places beyond Chromium- based browsers for a while now. urlpattern.spec.whatwg.org

I feel like a lot of people sleep on console commands beyond log. Group, table, and trace are all things I use regularly. Also, debugger is super 😎. I don't usually reach for what my IDE provides, but I might start after watching this www.youtube.com/live/gFcR8J9...

CSS is render- but not parser-blocking. This means that while a browser is fetching CSS, it is permitted to continue parsing the rest of your HTML but not render it. HOWEVER! If you’re not careful, you can inadvertently end up in parser-blocking territory.

I installed pyenv today and had trouble building on Ubuntu. Lo and behold there was this helpful gist that had exactly what I needed: gist.github.com/drconopoima/...

Alexander Krizhanovsky is now talking about scheduling HTTP streams fosdem.org/2025/schedul... fosdem.org/2025/schedul... #webperf #fosdem

✍️ I did an explainer on the CPU and GPU and how they coordinate tasks between them. Going to a series on rendering💥

I moved my personal site over to Cloudflare Pages over the weekend. It was really easy to get setup and I'm looking forward to using it more for some ideas I have.

This is my 2024 wrap-up post. For me, 2024 was a year of new jobs, transitions, and lots of learning. www.ethangardner.com/posts/2024-y...

This time off is just what I needed. 2024 kicked my ass, y'all.

The video of my talk from the Performance Now conference in Amsterdam was posted today. #perfnow

Web performance calendar day 9 with @ethangardner.com on how to relate performance efforts to our friends from ads, marketing and the C-level calendar.perfplanet.com/2024/putting...

I'm leading my team's meeting this month. For our fun activity, I took facts I knew about each person on the team and had AI draw an image of us as superheros. People loved trying to guess who they were in the picture and had to figure out what I knew about them. I would definitely recommend.

When I worked in media, I oversaw my org's cloud spend. This article hits home for me. calendar.perfplanet.com/2024/what-a-...

The Web Performance Calendar is off to a great start, with a great post from @nomster.bsky.social reminding us all why it's important to remember Goodhart's law.