diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4a21d4411..8c98e4c27 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -76,6 +76,20 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   opacity. Measured with the stylesheet delayed: 17–18 animating frames before,
   none after. Supersedes the `useMounted` approach in
   [#141](https://github.com/QuantEcon/quantecon-theme.mystmd/pull/141).
+- The WebKit FOUC guard no longer goes red for reasons unrelated to the critical
+  CSS. Hydration currently fails on every page load (React #418/#423), and the
+  recovery re-render restores the inlined critical `<style>` 150–240ms after
+  `DOMContentLoaded` — after the control test has deliberately stripped it from
+  the served HTML to prove the guard is meaningful. The measurement used to run
+  from the test after `domcontentloaded`, so on a loaded runner every control
+  assertion flipped at once. It now samples from an init script that fires
+  in-page on `DOMContentLoaded`, before hydration is even scheduled. Both cases
+  also assert their own preconditions — that the strip actually matched, and
+  whether the inline block is present as sampled — so a reshaped `CRITICAL_CSS`
+  reports as a stale strip pattern instead of as a run of confusing failures
+  about grid layout. The underlying hydration failure remains open in
+  [#126](https://github.com/QuantEcon/quantecon-theme.mystmd/issues/126);
+  carried over from [#141](https://github.com/QuantEcon/quantecon-theme.mystmd/pull/141).
 
 ### Dependencies
 - `@fontsource/pt-serif` 5.3.0 self-hosts the PT Serif heading face (400/700,
diff --git a/tests/visual/fouc.spec.ts b/tests/visual/fouc.spec.ts
index 8c61b8aac..58db8e327 100644
--- a/tests/visual/fouc.spec.ts
+++ b/tests/visual/fouc.spec.ts
@@ -27,7 +27,18 @@ import { test, expect, type Page, type Route } from "@playwright/test";
 
 const PAGE = "/";
 
+/**
+ * Aborts external stylesheets, optionally stripping the inline critical block.
+ *
+ * Returns a `strippedCritical` probe rather than trusting the regex to have
+ * matched. If `CRITICAL_CSS` is ever reshaped so the pattern stops matching,
+ * the control would otherwise run against a fully-styled page and fail on its
+ * *assertions* — reporting "expected block, received grid", which points at the
+ * critical CSS rather than at the stale pattern that is actually at fault.
+ * Asserting this probe turns that into a failure that names itself.
+ */
 async function isolateInlineCss(page: Page, { stripCritical = false } = {}) {
+  const probe = { strippedCritical: false };
   await page.route("**/*", async (route: Route) => {
     const req = route.request();
 
@@ -39,55 +50,114 @@ async function isolateInlineCss(page: Page, { stripCritical = false } = {}) {
     // Control: serve the document with the inlined critical <style> removed.
     if (req.resourceType() === "document" && stripCritical) {
       const response = await route.fetch();
-      const body = (await response.text()).replace(
+      const served = await response.text();
+      const body = served.replace(
         /<style>[^<]*:where\(\.simple-center-grid\)[^<]*<\/style>/g,
         "<!-- critical CSS removed for control -->",
       );
+      if (body !== served) probe.strippedCritical = true;
       return route.fulfill({ response, body });
     }
 
     return route.continue();
   });
+  return probe;
 }
 
-async function firstPaintState(page: Page) {
+/**
+ * Key under which the in-page sampler parks its measurement on `window`.
+ */
+const SAMPLE_KEY = "__qeFirstPaint";
+
+type FirstPaint = {
+  gridDisplay: string;
+  bodyFont: string;
+  sidebarRendered: boolean | null;
+  toggleCloseRendered: boolean | null;
+  backToTopOpacity: string | null;
+  appliedExternal: boolean;
+  criticalInline: boolean;
+};
+
+/**
+ * Measure at first paint, from inside the page, before React hydrates.
+ *
+ * The measurement used to run from the test after `domcontentloaded`, which
+ * left roughly a 150ms budget before hydration. That is not enough: hydration
+ * currently fails on every load (React #418/#423, tracked in #126), and the
+ * recovery re-render puts the critical `<style>` back — measured at 150–240ms
+ * after `DOMContentLoaded`. The control strips that block from the served HTML
+ * precisely to prove the guard is meaningful, so when React restored it the
+ * control's assertions all flipped at once and `fouc-guard` went red for
+ * reasons that had nothing to do with the critical CSS.
+ *
+ * Sampling from an init script closes that window: it runs on
+ * `DOMContentLoaded`, in-page and synchronously, and it reads only the DOM. The
+ * ordering is empirical rather than guaranteed — Remix v1 emits its entry as an
+ * inline `type="module" async` script whose imports must resolve before
+ * `entry.client.tsx` even schedules `requestIdleCallback`/`setTimeout` for
+ * `hydrateRoot`, and WebKit dispatches DOMContentLoaded at end of parse — but
+ * that ordering is what the #126 timings show, with the earliest observed
+ * restoration an order of magnitude later than the sample.
+ *
+ * Note this deliberately survives a *fixed* #126: a control that strips the
+ * inline block guarantees a hydration mismatch by construction, so no repair to
+ * the hydration failure itself could make a post-hydration sample safe here.
+ */
+async function firstPaintState(page: Page): Promise<FirstPaint> {
+  await page.addInitScript((key) => {
+    document.addEventListener(
+      "DOMContentLoaded",
+      () => {
+        const grid = document.querySelector(".simple-center-grid");
+        const sidebar = document.querySelector(".qe-toc");
+        const closeIcon = document.querySelector(".qe-toc-toggle__close");
+        const backToTop = document.querySelector(".qe-back-to-top");
+        const applied = Array.from(document.styleSheets).filter((sheet) => {
+          try {
+            return !!sheet.cssRules && sheet.cssRules.length > 0; // applied, not pending
+          } catch {
+            return false; // cross-origin / not yet loaded
+          }
+        });
+        const sample: FirstPaint = {
+          gridDisplay: grid ? getComputedStyle(grid).display : "(absent)",
+          bodyFont: getComputedStyle(document.body).fontFamily,
+          // A closed popover is `display: none` per the UA stylesheet, so it
+          // paints nothing even with no author CSS at all — hence both tests
+          // below expect it hidden.
+          //
+          // `null` when the element is missing, so a renamed hook fails the
+          // explicit presence guard instead of silently satisfying "not
+          // rendered".
+          sidebarRendered: sidebar ? sidebar.getClientRects().length > 0 : null,
+          // The toggle's close icon is hidden by a critical-CSS rule; without
+          // it both icons paint side by side on the first frame. `null` when
+          // missing, for the same reason as above.
+          toggleCloseRendered: closeIcon ? closeIcon.getClientRects().length > 0 : null,
+          // "Back to top" is hidden by opacity rather than display, and
+          // carries a transition — so if it paints visible here it will *fade*
+          // out once the stylesheet lands. The critical block pins it to 0.
+          // `null` when missing, as above.
+          backToTopOpacity: backToTop ? getComputedStyle(backToTop).opacity : null,
+          // null href == an inline <style>; any string == an external sheet
+          // that applied
+          appliedExternal: applied.some((sheet) => !!sheet.href),
+          // Whether the inline block is present in the document as sampled —
+          // the state the assertions below are actually about.
+          criticalInline: Array.from(document.querySelectorAll("style")).some((el) =>
+            (el.textContent ?? "").includes(":where(.simple-center-grid)"),
+          ),
+        };
+        (window as unknown as Record<string, unknown>)[key] = sample;
+      },
+      { once: true },
+    );
+  }, SAMPLE_KEY);
+
   await page.goto(PAGE, { waitUntil: "domcontentloaded" });
-  await page.waitForSelector(".simple-center-grid", { timeout: 5000 }).catch(() => {});
-  return page.evaluate(() => {
-    const grid = document.querySelector(".simple-center-grid");
-    const sidebar = document.querySelector(".qe-toc");
-    const closeIcon = document.querySelector(".qe-toc-toggle__close");
-    const backToTop = document.querySelector(".qe-back-to-top");
-    const applied = Array.from(document.styleSheets).filter((sheet) => {
-      try {
-        return !!sheet.cssRules && sheet.cssRules.length > 0; // applied, not pending
-      } catch {
-        return false; // cross-origin / not yet loaded
-      }
-    });
-    return {
-      gridDisplay: grid ? getComputedStyle(grid).display : "(absent)",
-      bodyFont: getComputedStyle(document.body).fontFamily,
-      // A closed popover is `display: none` per the UA stylesheet, so it paints
-      // nothing even with no author CSS at all — hence both tests below expect
-      // it hidden.
-      //
-      // `null` when the element is missing, so a renamed hook fails the
-      // explicit presence guard instead of silently satisfying "not rendered".
-      sidebarRendered: sidebar ? sidebar.getClientRects().length > 0 : null,
-      // The toggle's close icon is hidden by a critical-CSS rule; without it
-      // both icons paint side by side on the first frame. `null` when missing,
-      // for the same reason as above.
-      toggleCloseRendered: closeIcon ? closeIcon.getClientRects().length > 0 : null,
-      // "Back to top" is hidden by opacity rather than display, and carries a
-      // transition — so if it paints visible here it will *fade* out once the
-      // stylesheet lands. The critical block pins it to 0. `null` when missing,
-      // as above.
-      backToTopOpacity: backToTop ? getComputedStyle(backToTop).opacity : null,
-      // null href == an inline <style>; any string == an external sheet that applied
-      appliedExternal: applied.some((sheet) => !!sheet.href),
-    };
-  });
+  await page.waitForFunction((key) => !!(window as any)[key], SAMPLE_KEY, { timeout: 5000 });
+  return page.evaluate<FirstPaint, string>((key) => (window as any)[key], SAMPLE_KEY);
 }
 
 test.describe("FOUC guard (WebKit) — inline critical CSS styles the first paint", () => {
@@ -97,6 +167,10 @@ test.describe("FOUC guard (WebKit) — inline critical CSS styles the first pain
 
     // No external sheet applied, so anything styled below comes from inline CSS.
     expect(state.appliedExternal).toBe(false);
+    expect(
+      state.criticalInline,
+      "the inline critical <style> is missing from the page as served — app/root.tsx no longer inlines CRITICAL_CSS"
+    ).toBe(true);
     // The reported FOUC symptoms must be absent on first paint:
     expect(state.gridDisplay).toBe("grid"); // grid not collapsed to block
     // Head of the stack, not just a substring: "Source Sans 3 Variable" (the
@@ -125,9 +199,20 @@ test.describe("FOUC guard (WebKit) — inline critical CSS styles the first pain
   });
 
   test("control: removing the inline critical CSS reproduces the FOUC", async ({ page }) => {
-    await isolateInlineCss(page, { stripCritical: true });
+    const probe = await isolateInlineCss(page, { stripCritical: true });
     const state = await firstPaintState(page);
 
+    // Preconditions first, so a stale strip pattern reports as itself rather
+    // than as a run of confusing assertion failures about the critical CSS.
+    expect(
+      probe.strippedCritical,
+      "the strip pattern matched nothing in the served HTML — CRITICAL_CSS was reshaped and the regex in isolateInlineCss needs updating"
+    ).toBe(true);
+    expect(
+      state.criticalInline,
+      "the inline critical <style> is present despite the strip — it was restored before the sample (see #126)"
+    ).toBe(false);
+
     // With no CSS at all, the page is unstyled — this proves the guard above is
     // meaningful (the external abort really does strip styling).
     expect(state.appliedExternal).toBe(false);