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Security & Privacy Self-Review: moderate_viewport_heuristics

This document answers the questions from the W3C TAG Self-Review Questionnaire: Security and Privacy for the proposed moderate_viewport_heuristics extension to Speculation Rules.

Feature summary

Chromium's "moderate" eagerness speculation rules on mobile use a viewport-based heuristic to pick at most one on-screen link to prefetch/prerender each time the user stops scrolling. The heuristic has three tunable parameters:

• distance_from_pointer_down — a [low, high] band, as a fraction of screen height, gating how far a candidate link may be from the user's last pointerdown.
• largest_anchor_threshold — how much larger (by visible area) the largest candidate must be than the runner-up before it is chosen.

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Self-Review Questionnaire: Security and Privacy

Feature: Exposing redirectCount (and redirectStart / redirectEnd) on PerformanceNavigationTiming for TAO opted-in cross-origin redirect chains.

Specs:

• HTML: whatwg/html#12513
• Fetch: whatwg/fetch#1931
• Issue: w3c/navigation-timing#215

---

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"moderate" speculation rules eagerness proves very useful on desktop, but less so on mobile.

Chromium's current mobile heuristics are based on watching the viewport: when a link scrolls into view and looks like a plausible next click, the browser prefetches or prerenders it. This shipped in Chrome 138; there's more background in the blink-dev PSA.

"Looks like a plausible next click" comes down to three checks the browser runs

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Self-Review Questionnaire: Security and Privacy

styleDuration, layoutDuration, forcedStyleDuration, and forcedLayoutDuration attributes

Answers to the W3C Security and Privacy Self-Review Questionnaire for the newly added timing duration attributes in the Long Animation Frames API.

These attributes break down existing aggregate timing into finer-grained style and layout components:

• PerformanceLongAnimationFrameTiming.styleDuration / layoutDuration: Time spent in style recalculation and layout during the rendering phase of a long animation frame.
• PerformanceScriptTiming.forcedStyleDuration / forcedLayoutDuration: Time spent in synchronous (forced) style recalculation and layout triggered during script execution (e.g., by calling getComputedStyle() or getBoundingClientRect()).

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const totalStyleCost = async () => {
  return new Promise(resolve => {
    let total = 0;
    const obs = new PerformanceObserver(entryList => {
      const entries = entryList.getEntries();
      for (const entry of entries) {
          if (entry.paintTime) {
              total += entry.paintTime - entry.styleAndLayoutStart;
          } else {
              total += entry.startTime + entry.duration - entry.styleAndLayoutStart;

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Participants

Yoav Weiss

Rouslan Solomakhin (Google)

Darwin Yang (Google)

(Shopify)

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WICG: Address Autofill

• TPAC

Attendees: Westin, Martin Lechner, Yoav Weiss, Anne van Kesteren, Christian Indra, Rick Byers, Adam Rice, Michal Mocny, Dominic Batre

• Yoav: Discuss Address Autofill
• autofill is a key feature for login and web forms etc

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Joel: Welcome! Gonna show a live use case for FedCM

Demo to bootstrap the conversation

Shop.app is the buyer-facing identity at Shopify

Reaching checkout when you’re not authenticated can be complicated

Buyers that reach checkout authenticated, they’re more likely to successfully complete checkout

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<script type="importmap">
{
  "imports": {
    "square": "./module/shapes/square.js"
  },

  "integrity": {
    "./module/shapes/square.js": "sha384-oqVuAfXRKap7fdgcCY5uykM6+R9GqQ8K/uxy9rx7HNQlGYl1kPzQho1wx4JwY8wC"
  }
}

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Self-Review Questionnaire: Security and Privacy

This questionnaire has moved.

1. What information does this feature expose, and for what purposes?

No Information is exposed.

2. Do features in your specification expose the minimum amount of information