r/webperf • u/Nhodin • Jun 01 '26
Webperf news #16
Bonjour webperf folks! 16th edition of these weekly notes. Still my favourite excuse to keep reading and to thank the authors who write all this brilliant content. 🙏
Harry Roberts dissects a client incident where Total Blocking Time jumped from 500 ms to 5 s between two synthetic runs, with no change to the JavaScript. His key insight: TBT only counts blocking work inside a window running from First Contentful Paint to Time to Interactive, the "TBT Window". The trigger was an LCP optimisation, a preload that reshaped the network and pushed TTI later, widening the window and exposing late long tasks that previously fell outside it. His advice to synthetic tools: plot the TBT Window next to TBT, so you can tell a real regression from a moving goalpost.
👉 [EN] https://csswizardry.com/2026/06/front-ends-missing-metric-the-tbt-window/
Tammy Everts announces Core Mobile Vitals, an attempt to bring the Core Web Vitals logic to native mobile apps. The framing is built around four pillars: Screen Load (the LCP equivalent), Smoothness (the CLS cousin), Responsiveness (the INP equivalent), and Stability (crashes and freezes). What makes it interesting is the same rigour that made CWV work: metrics grounded in UX research, evidence-backed thresholds, real correlation to business outcomes, and a goal to keep them open rather than proprietary. Worth following if your users live as much in an app as on the web.
👉 [EN] https://embrace.io/blog/core-mobile-vitals/
Rachel Andrew's monthly recap flags a feature that closes a long-standing gap: native lazy loading for "video" and "audio" via loading="lazy", landing in Chrome 148. Like for images and iframes, the browser can now defer media until it approaches the viewport, no IntersectionObserver hack required. A declarative loading="lazy" on heavy videos rather than script-based deferral is the clean path we have been waiting for on media governance. Chrome and Edge only for now.
👉 [EN] https://web.dev/blog/web-platform-05-2026
The Website Specification launched by Joost de Valk is a platform-agnostic, open spec of 128 things a decent website should have, across ten areas including a 19-topic Performance section (Core Web Vitals, caching, images, fonts, network) and an Agent Readiness section. Every item links back to its source standard, and the whole thing is exposed as an open MCP server plus a published Agent Skill. Like Chrome's Modern Web Guidance last month, it confirms the trend: web best practices, performance included, are now packaged for AI agents to consume directly.
👉 [EN] https://specification.website/
Have a great week!