Joan León - English

A 301 on every click: the trailing slash that slowed down all internal navigation

Sun, 19 Jul 2026 00:00:00 GMT

A real finding on skills.addy.ie: every internal link triggered a 301 redirect on Netlify because of a missing trailing slash. What it costs the UX, how I spotted it in DevTools, the quick fix I first thought of, and the underlying solution (Astro directory format + trailingSlash: 'always'). With Network panel evidence before and after.

Keep-Alive is not Cache-Control: anatomy of a network waterfall

Thu, 02 Jul 2026 10:00:00 GMT

A real audit case: two heavy images (608 KB and 403 KB) over HTTP/1.1 with Connection: Keep-Alive but no Cache-Control. Why they are different things, what HTTP/2 fixes and how preconnect gets ahead of the handshake. With an interactive waterfall demo.

The mystery of the self-repainting #document: Paint flashing and composited layers

Sun, 21 Jun 2026 10:00:00 GMT

Why the #document layer racks up hundreds of repaints at rest because of a ticker animated with transform, how to catch it with the Layers tab and Paint flashing, and how to promote the element to its own composited layer with will-change.

Antigravity Skills: advanced YAML frontmatter configuration

Sat, 13 Jun 2026 10:00:00 GMT

The kind, model, temperature, max_turns and tools fields in SKILL.md frontmatter let you precisely control how each agent runs in Antigravity. A practical guide to the extended fields that Google hasn't officially documented yet.

Video editing in the browser with WebGPU

Tue, 28 Apr 2026 10:00:00 GMT

WebGPU enables real-time video effects and transformations in the browser. Practical implementation with benchmarks and real-world limitations.

Forced Synchronous Layout: how to detect and analyze it with DevTools

Mon, 27 Apr 2026 10:00:00 GMT

How a scroll reset after mass DOM manipulation triggers a Forced Synchronous Layout, how to detect it in the Performance Panel and Selector Stats in Chrome DevTools, and how to fix it with requestAnimationFrame.

ML in the browser with WebGPU: real-time inference

Tue, 21 Apr 2026 10:00:00 GMT

How to use WebGPU as the TensorFlow.js backend to run ML models in the browser with real performance. Benchmarks and practical use cases.

Automate the browser from the terminal with Chrome DevTools MCP CLI

Thu, 16 Apr 2026 10:00:00 GMT

The chrome-devtools-mcp package includes an experimental CLI that lets you control Chrome directly from the terminal: navigate, interact, audit performance, analyze network and memory, all without leaving the shell.

Image processing in the browser with WebGPU

Tue, 14 Apr 2026 10:00:00 GMT

How to use WebGPU to apply filters and transformations to images in the browser. Real benchmarks comparing with Canvas 2D.

WebGPU: the browser's new performance engine

Tue, 07 Apr 2026 10:00:00 GMT

WebGPU exposes the GPU directly to the browser, opening new possibilities for intensive processing: images, video, ML, and more.

AI in Web Performance Audits: Deterministic Measurement and Actionable Reports

Thu, 26 Mar 2026 10:00:00 GMT

How to combine deterministic scripts and AI agents to go from metrics collection to an actionable report, and lay the foundations for continuous regression testing in performance.

AI-Driven Performance Debugging: Gemini + Chrome DevTools MCP

Wed, 18 Mar 2026 10:00:00 GMT

A hands-on workshop for performance debugging with Gemini CLI connected to the browser via Chrome DevTools MCP: from identifying a Long Task in minified code to having the agent create the branch and open the PR.

Adding an AI summary to blog posts with the Chrome Summarizer API

Fri, 13 Mar 2026 11:10:58 GMT

How to add an AI summary button to an Astro blog using the browser's native Chrome Summarizer API, without servers or external APIs.

WebPerf Snippets: progressive disclosure in Claude Skills

Tue, 10 Mar 2026 10:00:00 GMT

Applying Anthropic's progressive disclosure system (L1/L2/L3) to reduce the context loaded by WebPerf Snippets Skills and free space in the agent's context window.

WebPerf Snippets agent-first: structured returns and minimal tokens

Sun, 08 Mar 2026 10:00:00 GMT

WebPerf Snippets scripts now return structured JSON directly from evaluate_script() and are minified at build time with dead code elimination, significantly reducing token footprint.

WebPerf Snippets and Agent SKILLs: deterministic audits with Chrome DevTools MCP

Wed, 04 Mar 2026 10:00:00 GMT

WebPerf Snippets become Agent SKILLs with scripts as deterministic tools for AI agents, executable directly in the browser via Chrome DevTools MCP.

WebPerf Snippets + WebMCP: web metrics that AI understands

Tue, 03 Mar 2026 10:00:00 GMT

WebPerf Snippets exposes its performance measurement tools via WebMCP, the new protocol that lets AI assistants interact with the browser natively.

WebAssembly: the W3C's forgotten standard

Sun, 01 Mar 2026 10:00:00 GMT

WebAssembly has been a W3C standard since 2019, but many developers still don't know about it. What it really is, how it works, and why you should know about it.

SVGs on the web: performance comparison based on how you load them

Tue, 24 Feb 2026 10:00:00 GMT

Inline, sprite with , , ... each way of loading an SVG has different implications for performance, caching and rendering. A practical analysis to help you choose well.

Practical guide to the element: from the basics to LCP

Sun, 22 Feb 2026 10:00:00 GMT

The element has attributes that make a real difference in performance: loading, decoding, fetchpriority, srcset and sizes. A practical guide to using them well.

Yield to Main: setTimeout vs queueMicrotask vs scheduler.postTask

Wed, 18 Feb 2026 10:00:00 GMT

A practical guide on how to use setTimeout, queueMicrotask and scheduler.postTask to break up long tasks and optimize INP. Comparison with measurable examples.

Optimizing Consent Scripts for Core Web Vitals

Fri, 13 Feb 2026 10:00:00 GMT

Consent management scripts (CMP) compete with critical resources if not prioritized correctly. A server-side strategy to optimize loading based on visit type.

Chrome DevTools for Debugging Web Performance

Fri, 02 Jan 2026 00:00:00 GMT

This article is available in English on the original source at PerfPlanet Performance Calendar 2025.

Optimize rendering with content-visibility

Sat, 27 Dec 2025 00:00:00 GMT

The CSS property content-visibility improves rendering performance. Practical case with metrics and snippets to detect opportunities.

CSS Media Queries Analysis to Optimize Performance

Tue, 02 Dec 2025 00:00:00 GMT

Learn how to analyze CSS media queries to detect and optimize unused styles on mobile, improving your website's performance with a simple JavaScript snippet.

Talks 2023

Sun, 31 Dec 2023 00:00:00 GMT

A review of the talks I gave during 2023

Interpret Your Metrics Correctly

Sat, 04 Mar 2023 00:00:00 GMT

A metric number is just a number, until you know how to interpret it.

Analyzing User Flow Web Performance with Chrome DevTools

Mon, 08 Nov 2021 00:00:00 GMT

In Chrome 97 a new tool appears in DevTools to measure performance in the navigation and interaction experience of visitors to our website

My Opinion on Tailwind CSS

Tue, 26 Jan 2021 00:00:00 GMT

Utility-First CSS frameworks are gaining a lot of traction, especially Tailwind CSS. Is it a good option?

I Prefer PostCSS

Fri, 08 Jan 2021 00:00:00 GMT

Let me explain why I prefer PostCSS for writing CSS.

It's Curious How Forgotten and Mistreated CSS Is

Mon, 04 Jan 2021 00:00:00 GMT

At over 24 years old, CSS continues to be a despised language.

Detect AVIF Support for Your CSS Images

Sun, 06 Dec 2020 00:00:00 GMT

Implement the new AVIF image format in your CSS `background-image`.

CSS Books

Sat, 17 Oct 2020 00:00:00 GMT

A compilation of CSS books that can help you improve your knowledge in the art of styling your projects.

Learning to Program with Processing and P5js

Sat, 12 Sep 2020 00:00:00 GMT

There are many methodologies and languages for learning to program, which is why the first thing a person who wants to learn to program faces is choosing a language or development platform.

Agile Design with TDD

Mon, 24 Aug 2020 00:00:00 GMT

In this article I want to share my opinion of the book 'Agile Design with TDD', from which I've been able to learn the foundations of TDD.

Clean JavaScript: Clean Code, SOLID and Testing

Wed, 19 Aug 2020 00:00:00 GMT

I want to recommend the book Clean JavaScript, by Miguel A. Gómez, where he teaches us to apply good practices, write robust and easy-to-maintain code. With the aim of developing good software with JavaScript.

CSS Animations

Sun, 12 Jul 2020 00:00:00 GMT

With CSS animations we have much more control than with transitions to create animations thanks to @keyframes.

CSS Transitions

Sun, 23 Feb 2020 00:00:00 GMT

CSS Transitions allow property value changes to occur progressively over a specified period of time.

AVIF, the Definitive Image Format?

Mon, 10 Feb 2020 00:00:00 GMT

AVIF is a next-generation image format that reduces JPEG images by up to ~50%.

Web Animation

Mon, 10 Feb 2020 00:00:00 GMT

We've had animation on the web for a long time, let's take a look at the different ways to do web animation.

Finally Understanding JPG

Tue, 04 Feb 2020 00:00:00 GMT

This article is available in English on the original source by Christoph Erdmann.

CSS Pixel Art

Sun, 12 Jan 2020 00:00:00 GMT

Pixel Art is a design technique that continues to be used as a resource in many video games. With CSS we can use this technique in a very simple way.

CSS Houdini, More Than Magic

Sat, 04 Jan 2020 00:00:00 GMT

For many people CSS is a magician's thing, some might even believe it's witchcraft, but that's because they don't know the necessary spells to have real control over CSS.

Motion Craftsmanship

Sat, 04 Jan 2020 00:00:00 GMT

Little needs to be said about the importance of animation in websites and applications, designers and developers are aware of it, just by opening the browser or applications we can appreciate it. But let's see what's hidden under the definition of Motion Craftsmanship.

Where to Start Optimizing My Images

Sat, 04 Jan 2020 00:00:00 GMT

As web development professionals, we worry about choosing the best framework, the most current library, applying best practices in JavaScript and CSS, being careful with HTML semantics to improve SEO, improving our content's accessibility, bundling and splitting files to optimize loading, but what about images?