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fix: use renderBlockingStatus to avoid false positives in Critical CSS Detection #65

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Merged
nucliweb merged 1 commit into from
Mar 10, 2026
Merged

fix: use renderBlockingStatus to avoid false positives in Critical CSS Detection #65

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4 changes: 2 additions & 2 deletions .claude/skills/webperf-loading/scripts/Critical-CSS-Detection.js
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4 changes: 2 additions & 2 deletions skills/webperf-loading/scripts/Critical-CSS-Detection.js
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26 changes: 18 additions & 8 deletions snippets/Loading/Critical-CSS-Detection.js
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Original file line number Diff line number Diff line change
Expand Up @@ -24,15 +24,25 @@
const href = link.href;
const media = link.getAttribute("media") || "all";

// A stylesheet is render-blocking when it applies to screen without deferral
const isRenderBlocking =
media === "all" ||
media === "" ||
media === "screen" ||
(media.toLowerCase().includes("screen") &&
!media.toLowerCase().includes("print"));

const perfEntry = perfEntries.find((e) => e.name === href);

// Prefer browser-reported render-blocking status (PerformanceResourceTiming API,
// Chrome 107+). This correctly handles the media="print" onload="this.media='all'"
// deferral pattern — by page load time the media attribute has already changed to
// "all", so a DOM-only check would produce false positives.
let isRenderBlocking;
if (perfEntry && perfEntry.renderBlockingStatus !== undefined) {
isRenderBlocking = perfEntry.renderBlockingStatus === "blocking";
} else {
// Fallback for browsers without renderBlockingStatus support.
// Note: may produce false positives when the media/onload deferral pattern is used.
isRenderBlocking =
media === "all" ||
media === "" ||
media === "screen" ||
(media.toLowerCase().includes("screen") &&
!media.toLowerCase().includes("print"));
}
const transferSize = perfEntry ? perfEntry.transferSize : 0;
const decodedSize = perfEntry ? perfEntry.decodedBodySize : 0;

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