diff --git a/app/root.tsx b/app/root.tsx
index 7e9cce443..e86135e03 100644
--- a/app/root.tsx
+++ b/app/root.tsx
@@ -42,13 +42,29 @@ export const meta: V2_MetaFunction<typeof loader> = ({ data }) => {
  * Critical CSS — inlined in <head> to fix the Safari/WebKit FOUC on navigation
  * (https://github.com/QuantEcon/quantecon-theme-src/issues/66).
  *
- * Static builds (`myst build --html`) navigate via full document loads, and
- * WebKit paints the freshly-navigated document for ~1 frame BEFORE any <link>
- * stylesheet applies — even the same-origin Tailwind app.css. That frame shows
- * the default serif font and the content grid collapsed to `display: block`
- * (i.e. the "raw HTML" flash users report). An inline <style> is parsed
- * synchronously with the document, so it styles that very first paint with no
- * network round-trip.
+ * The mechanism is NOT a pre-stylesheet first paint. WebKit holds first paint
+ * until the render-blocking <head> stylesheets apply — probed with a
+ * rAF-from-document-start sampler: no frame renders unstyled on the static
+ * build, cold or warm cache, even with every CSS response delayed 800ms. The
+ * unstyled frame arrives ~200ms AFTER first paint, when React hydration fails
+ * (minified #418/#423) and the recovery client render re-patches whatever
+ * diverged between server and client markup. A divergence in the <head> makes
+ * that pass re-create head nodes (#126 measured it re-inserting a missing
+ * <style> at 195ms) — and a re-inserted stylesheet <link> re-applies
+ * asynchronously, while a re-inserted inline <style> applies the instant the
+ * node lands. In that gap this block is the only styling on the page, which
+ * is the styled -> unstyled -> styled flicker users reported as the "raw
+ * HTML" flash. A body-level mismatch, by contrast, recovers without touching
+ * the head at all (probed by injecting a stray node into the served <body>:
+ * both errors fire, zero head mutations, no flash).
+ *
+ * Current builds hydrate cleanly — no #418/#423 on any fixture page, dev or
+ * static — so day to day this block is the safety net for whenever the
+ * external stylesheets are absent, however that comes about; the FOUC guard
+ * test simulates that state by aborting them. The practical rule stands
+ * regardless of mechanism: anything that must not flash (hidden by opacity,
+ * gated by a transition) needs a rule HERE — at flash time React is already
+ * mounted, so mount-gating cannot help, and only this inline CSS survives.
  *
  * Every selector is wrapped in `:where(...)` so these rules carry **zero**
  * specificity: they take effect only while nothing else has loaded, and the
diff --git a/tests/visual/fouc.spec.ts b/tests/visual/fouc.spec.ts
index 58db8e327..72abebbc4 100644
--- a/tests/visual/fouc.spec.ts
+++ b/tests/visual/fouc.spec.ts
@@ -3,11 +3,15 @@ import { test, expect, type Page, type Route } from "@playwright/test";
 /**
  * FOUC guard (WebKit) — QuantEcon/quantecon-theme-src#66.
  *
- * In the static build every navigation is a full document load, and WebKit
- * paints the fresh document *before* its external `<link>` stylesheets apply —
- * so for ~1 frame the page renders with the default serif font and the content
- * grid collapsed to `display: block`. The fix inlines critical CSS into `<head>`
- * (see `app/root.tsx`), which parses synchronously and styles that first paint.
+ * The flash this guards against is NOT a pre-stylesheet first paint — WebKit
+ * holds first paint until the `<head>` stylesheets apply. It is the window
+ * where those stylesheets stop applying after load: historically, React's
+ * hydration-recovery re-render re-creating head nodes when server and client
+ * markup diverged there, which re-applies a `<link>` asynchronously but an
+ * inline `<style>` synchronously (see the CRITICAL_CSS comment in
+ * `app/root.tsx`, and #126 for the measurements). In that window the page
+ * shows the default serif font and the content grid collapsed to
+ * `display: block` unless the inlined critical CSS covers it.
  *
  * The contents drawer is checked here too: any panel that relies on author CSS
  * to stay hidden paints open in that same frame. It is a popover now, so the UA
@@ -21,8 +25,9 @@ import { test, expect, type Page, type Route } from "@playwright/test";
  * assertion below fails. The control case strips the inline block to prove the
  * abort genuinely removes external styling (otherwise the guard would be moot).
  *
- * Runs in the `webkit-fouc` Playwright project only — Chromium paint-holds and
- * cannot exhibit this flash.
+ * Runs in the `webkit-fouc` Playwright project only — the flash was only ever
+ * observed in Safari/WebKit (quantecon-theme-src#66), and this suite's
+ * abort-the-stylesheets simulation exercises the guard there.
  */
 
 const PAGE = "/";
