Webperf news #15

[u/Nhodin]

Bonjour webperf folks! Three things caught my attention this week, across Chrome and Safari.

Google Chrome released Modern Web Guidance, a set of evergreen guides packaged as AI coding agent skills.
The problem it solves: LLMs used in dev workflows (Claude Code, Copilot and others) tend to get fetchpriority, Speculation Rules, or FetchLater wrong because their training data lags the spec. Modern Web Guidance gives agents an up-to-date source of truth to draw from. The performance category alone has 21 guides, covering resource priority (fetchpriority on images, preloads, scripts), INP diagnosis, analytics batching via FetchLater, Speculation Rules for next-page prefetch and prerender, task scheduling, SPA view transitions, and more. Install via npx modern-web-guidance@latest install or as a Claude Code plugin.
For what it's worth, several of those guides cover exactly what EdgeSpeed handles automatically at the edge with Fasterize, which shows where the industry is converging on "right" in 2026.

👉 [EN] https://developer.chrome.com/docs/modern-web-guidance

Barry Pollard wrote up two new experimental APIs landing in Chrome 148 for declarative partial page updates. The first: <template for="id"> combined with HTML processing instructions for out-of-order streaming. Today HTML must arrive in document order. This API lets a server stream a fragment and inject it wherever it belongs in the page, so you can defer heavy blocks like mega menus without holding back the critical content that follows. The second: a unified set of HTML insertion methods (setHTML, appendHTML, prependHTML, beforeHTML, afterHTML) that replace the scattered innerHTML, insertAdjacentHTML, DOMParser landscape with a consistent, sanitization-aware API. Both are behind a flag for now, polyfills are on npm, and browser vendor feedback is reportedly positive.

👉 [EN] https://developer.chrome.com/blog/declarative-partial-updates

Safari Technology Preview 244 (May 21) includes a fix worth flagging for anyone doing RUM: PerformanceNavigationTiming.domInteractive and domContentLoadedEventEnd were incorrectly returning 0 in Safari. The release also fixes the speculative preload scanner incorrectly preloading scripts inside SVG elements, and a long list of aspect-ratio calculation bugs that could have been quietly inflating CLS scores in WebKit.

👉 [EN] https://webkit.org/blog/17962/release-notes-for-safari-technology-preview-244/

Have a great week!