Harry Roberts

csswizardry.gumroad.comSubscribeI’ve been writing or creating content in some form or another since 2009. Some of you have been with me since the very beginning. That over 15 years!The articles and research I publish on csswizardry....

8 Aug 2025

🧑‍💻 I wasn’t going to mention this until next week but it’s Friday—treat yourself! I’ve just launched a members-only subscription: screencasts, Q&A, mentoring, discounts, and more. 👉 csswizardry.gumroad.com/l/subscribe

issues.chromium.orgChromium

I’ve spotted a @developer.chrome.com bug a few times in the wild, but today I finally caught it in the act. Pages hanging for 5+ minutes! CrBug raised here: issues.chromium.org/issues/57632...

@tunetheweb.com repro’d it in no time! Spite is a great motivator…

Two of my favourite opening slides from my ongoing work with LEGO. Sometimes, site-speed really is a *them* problem. It’s a function of your work and your audience, and one, both, or neither can move the needle.

One of the healthiest things a developer can do when faced with a bug or regression is doubt themselves first, but sometimes it’s something completely out of your hands.

github.blogImproving site performance by shipping more CSSHow we fully migrated github.com away from CSS-in-JS.

It’s a real shame that folk a decade ago weren’t warning developers against utterly insane things like CSS-in-J — oh.

GitHub@github.com

GitHub.com has fully moved off CSS-in-JS to CSS Modules, resulting in 55% less server-side render time and 25% faster component init. https://github.blog/engineering/architecture-optimization/improving-site-performance-by-shipping-more-css/

🇬🇧 ✈️ 🇳🇱 Just landed in Schiphol ready for @fronteers.bsky.social Dark Mode tomorrow!

csswizardry.comWeb-Perf Wednesday 011 – SpeedCurve’s INP Observer Missed Its Threshold – CSS WizardryA long-standing lux.js bug left Event Timing on its 104 ms default, while browser releases improve testing and rendering performance.

#WebPerfWednesday 11 is out, in which we fix a longstanding bug in @speedcurve.com, and Safari gets network throttling! csswizardry.com/2026/09/web-...

Tomorrow, I’m starting a new project with LEGO. Look how excited I am!

csswizardry.comTime to First Byte: What It Is and How to Improve It – CSS WizardryJust how much does TTFB matter when it comes to front-end performance?

17 Sept

What is Time to First Byte and what can you do about it? csswizardry.com/2019/08/time...

csswizardry.comA Layered Approach to Speculation Rules – CSS WizardryThe new Speculation Rules API is incredibly powerful, but we can do so much more! By taking a layered approach, we can add more progressive functionality.

14 Sept

Are you getting the most out of your Speculation Rules? Probably not! csswizardry.com/2024/12/a-la...

csswizardry.comFront-End’s Missing Metric: The TBT Window – CSS WizardryThe TBT Window is the FCP-to-TTI interval used to calculate Total Blocking Time. If FCP or TTI moves, TBT can change even when long tasks do not.

11 Sept

If you’re tracking your Core Web Vitals with synthetic, you probably also want to track your Total Blocking Time window: csswizardry.com/2026/06/fron...

csswizardry.comCSS Wizardry Blog Archives – CSS WizardryBrowse the full CSS Wizardry archive of articles on web performance, Core Web Vitals, caching, CSS architecture, and front-end engineering.

10 Sept

Over the last day or so, Codex and I have been working on adding a proper taxonomy and associated categories and tags to the blog. Finally, @csswizardry.com has a proper archive! csswizardry.com/blog/

10 Sept

The reason this has been absent for so long is that @csswizardry.com is built on Jekyll which has no first class support for categories and tags, so I’m especially proud of this: csswizardry.com/blog/tags/

csswizardry.comBetter Browser Caching with No-Vary-Search – CSS WizardryNo-Vary-Search lets HTTP caches ignore irrelevant query parameters such as UTM tags, while still keeping meaningful ones like product variants in the cache key.

10 Sept

Another day, another compelling use case for `No-Vary-Search`! csswizardry.com/2026/05/bett...

9 Sept

I have ChatGPT read my `.vimrc` and send me a daily #Vim tip to read with my morning coffee.

4 Sept

I’ve always had a very good habit of `[TAGGING]` things like calendar events or Jira titles and the like — it makes it so much easier to find arbitrary topics with simple grep or search. It turns out it’s *incredibly* useful in this agentic world!

I’ve been using @talat.app for a few months now and it’s completely changed how I handle meetings. I’ll prime Codex for an upcoming call and what I need out of it, then point it at the Talat MCP and get it to extract the bits that are meaningful.

It solves my two biggest on-meeting problems: 1) being distracted in front of a client as I take notes, and 2) not taking good enough notes as I still need to be present.

Anyway. This is #notAnAd! I have a fully paid up license and I can literally show you receipts, but! I am also informally giving away one free lifetime license to ONE person. Follow me and @talat.app and quote-post this post with how it would help you.

I added BCP 14 rules to `AGENTS.md`. Let’s see if/how that helps when writing spec-like prompts.

rfc-editor.orgBCP 14 subseries contains 2 RFCs | RFC EditorBCP14 contains RFC 2119, RFC 8174.

BCP 14 is the practice that collates and defined MUST, SHOULD, MAY, etc. www.rfc-editor.org/info/bcp14/

csswizardry.comWeb-Perf Wednesday 006 – Faster Browser Releases Change Your RUM Population – CSS WizardryChrome and Firefox are moving to two-week releases, making browser cadence part of how teams interpret RUM and plan performance testing.

26 Aug

🗓️ Published! This week’s #webPerfWednesday looks at Chome’s new release cadence, Unattributed Navigation Overhead, and soft navs in DevTools: csswizardry.com/2026/08/web-...

22 Aug

Today, I walked around Costco snapping photos of meat labels. I slung them all into my own personal chef GPT, Pierre. 👈 This was my input and 👉 That is their output. For <£400 all in, I’m eating like a king five days a week up until 19 November.

csswizardry.comHow to Find an Exact Gmail Thread with Codex or Claude – CSS WizardryUse Gmail’s Message-ID to point Codex or Claude at one exact email and retrieve the complete conversation rather than a plausible result.

22 Aug

The £400 includes the best-guess estimate for supporting ingredients. So only an additional £300 until mid November for the whole menu.

Dry-run. Unreal.

I wrote this up a bit more formally: csswizardry.com/2026/08/how-...

Harry Roberts@csswizardry.com

Instead of asking Codex to look in Gmail for ‘[subject]’ (risky and prone to duplicates or misinterpretation), head to ‘Show original’ and use the RFC 822 `MESSAGE-ID` (from 1982!) to target an exact specific message. From there, it can surface the entire visible thread.

Inb4 ‘you doxxed yourself!!’: that’s a nonexistent mailbox and the ID is the Base64 for a transparent GIF.

csswizardry.comIntroducing LUX Sidecar: Your SpeedCurve Companion – CSS WizardryLUX Sidecar is a small SpeedCurve RUM plugin that adds browser and navigation context to your real-user performance data.

20 Aug

If you use @speedcurve.com—and you should—then LUX Sidecar is for you! csswizardry.com/2026/08/intr...

csswizardry.comUnattributed Navigation Overhead (UNO) Is Underrated – CSS WizardryUnattributed Navigation Overhead exposes the part of Time to First Byte that Navigation Timing cannot explain, including hidden cross-origin redirects.

19 Aug

Unattributed Navigation Overhead is one of the biggest things you’re missing out on… csswizardry.com/2026/08/uno-...

csswizardry.comWeb-Perf Wednesday 005 – RUM Needs More Than a Percentile – CSS WizardryLUX Sidecar adds context to RUM, while browser-tool fixes show why measurement conditions and small debugging details matter.

19 Aug

#webPerfWednesday no. 5 is live! We look at DevTools bug fixes that landed in preview versions of Chrome and Safari, as well as new streamable `fetch` uploads (confusing): csswizardry.com/2026/08/web-...

csswizardry.comWhen Safari’s ‘Ignore Cache’ Ignored Cache-Control – CSS WizardrySafari Technology Preview 250 fixed a subtle Web Inspector bug that overwrote page-authored Cache-Control request headers when the resource cache was disabled.

17 Aug

🐛 Safari published an unusually terse bug fix in STP 250 — I looked into It. csswizardry.com/2026/08/when...

gist.github.comLUX Sidecar – Your SpeedCurve CompanionLUX Sidecar – Your SpeedCurve Companion. GitHub Gist: instantly share code, notes, and snippets.

Friday freebie for all you @speedcurve.com fans: LUX Sidecar – Your SpeedCurve Companion gist.github.com/csswizardry/...

I’ve started including a `mission-statement.md` alongside my project-level `AGENTS.md`. It contains philosophical notes about the project: why it exists, what questions it exists to answer, what ‘good’ looks like, etc.

`AGENTS.md` is then told ‘where exact directions are unspecified or unclear, ensure the work aligns with `mission-statement.md`’ as a bit of a fallback/vibe-check. It’s not failsafe or bulletproof, but I have noticed it being invoked a few times recently and… it works!

The mission statement should never include instruction or implementation detail—it should be a guiding principle. The end that the means work around.

csswizardry.comLow- and Mid-Tier Mobile for the Real World (2026) – CSS WizardryThe 2026 update to my real-world device testing recommendations paints a clearer picture than ever…

21 July

🤳 I just published the 2026 update to my ongoing research, ‘Low- and Mid-Tier Mobile for the Real World’. What are this year’s target hardware? Find out! csswizardry.com/2026/07/low-...

15 July

Has anyone written/built an `llms.txt` that they’re really happy with? And have you noticed it working/helping? I recently updated my research agent to check all websites for `/llms.txt` before diving straight into parsing pages of HTML and I’m looking forward to seeing if/how effective it is.

8 July

If you’re doing any reasonable amount of performance work, ensure you have a fresh browser profile with no ad blockers installed, and ensure this is unchecked (Chrome)! You’re only getting part of the picture otherwise.

29 June

For the last month or so, I’ve been playing Wordle against ChatGPT and keeping track of our scores. It’s incredibly close, but I’m just winning.

29 June

I solve the day’s puzzle myself, then when I’ve completed it, I walk ChatGPT through its game by playing on its behalf and pasting screenshots in. It’s fun!

matuzo.atContext-aware headings in HTML - Manuel MatuzovicI’m a frontend developer, consultant, accessibility auditor, blogger, teacher, and book author from Austria.

26 June

A great, succinct, informative write up from my pal @matuzo.at! Context-aware headings in HTML: www.matuzo.at/blog/2026/co...

It’s true…

Today, I spoke at the inaugural #DUNC26 in Hull—a real gem of a city. The conference packed so much diversity of content into a one-day, one-track event, BUT! My favourite takeaway is that every single speaker listed a personal site/blog in their slides. I visited every one of them. What a day.

2 June

🔍 I can’t help but feel like we’re missing something… Front-End’s Missing Metric: csswizardry.com/2026/06/fron...

meetup.comWeb Performance Special — with Harry Roberts & Leaning Technologies, Tue, May 26, 2026, 6:00 PM | MeetupJoin us this May for a special Leeds JS event focused entirely on **web performance** — one of the most critical (and often overlooked) aspects of modern web development. W

26 May

🎤 I’m speaking at #LeedsJS TONIGHT! Free to attend. Be there or be aspect-ratio: 1; www.meetup.com/leeds-js/eve...

27 May

Woohoo! That was fun. And so nice to give a talk on home turf.

13 May

🩺 Meet Your Users Where They Are with Obs.js csswizardry.com/2026/05/meet...