diff --git a/src/help-window.ts b/src/help-window.ts
index 4da2d80..f8dd7de 100644
--- a/src/help-window.ts
+++ b/src/help-window.ts
@@ -4,7 +4,6 @@ import { getHelpCatalog, type HelpCatalog } from "./help-content-localized";
 import { renderHelpMath } from "./help-math";
 import {
   getResolvedLocale,
-  isLanguagePreference,
   setLanguagePreference,
   t,
   type LanguagePreference,
@@ -16,13 +15,27 @@ interface HelpWindowPayload {
   theme: AppTheme;
 }
 
+function queryLanguagePreference(value: string | null): LanguagePreference | null {
+  switch (value) {
+    case "system": return "system";
+    case "en": return "en";
+    case "zh-CN": return "zh-CN";
+    case "zh-TW": return "zh-TW";
+    case "ja": return "ja";
+    case "es": return "es";
+    case "fr": return "fr";
+    case "de": return "de";
+    default: return null;
+  }
+}
+
 export class HelpWindowApp {
   private catalog: HelpCatalog;
   private activeSection = "";
 
   constructor(private readonly root: HTMLElement) {
-    const requestedLanguage = new URLSearchParams(window.location.search).get("lang");
-    if (requestedLanguage && isLanguagePreference(requestedLanguage)) setLanguagePreference(requestedLanguage);
+    const requestedLanguage = queryLanguagePreference(new URLSearchParams(window.location.search).get("lang"));
+    if (requestedLanguage) setLanguagePreference(requestedLanguage);
     this.catalog = getHelpCatalog(getResolvedLocale());
     this.activeSection = this.catalog.sections[0]?.id ?? "";
     root.addEventListener("contextmenu", (event) => event.preventDefault());
diff --git a/src/main.ts b/src/main.ts
index 3d2401f..aeb2716 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -8,7 +8,16 @@ if (!root) throw new Error(t("error.rootMissing"));
 const appRoot = root;
 
 function showFatalError(error: unknown): void {
-  appRoot.innerHTML = `<main class="fatal-error"><h1>${t("runtime.fatalTitle")}</h1><p>${String(error)}</p><small>${t("runtime.fatalHint")}</small></main>`;
+  const fatal = document.createElement("main");
+  fatal.className = "fatal-error";
+  const title = document.createElement("h1");
+  title.textContent = t("runtime.fatalTitle");
+  const detail = document.createElement("p");
+  detail.textContent = String(error);
+  const hint = document.createElement("small");
+  hint.textContent = t("runtime.fatalHint");
+  fatal.append(title, detail, hint);
+  appRoot.replaceChildren(fatal);
 }
 
 try {
diff --git a/src/statistics-chart.ts b/src/statistics-chart.ts
index 1b112d2..11105a6 100644
--- a/src/statistics-chart.ts
+++ b/src/statistics-chart.ts
@@ -364,7 +364,7 @@ export class StatisticsCharts {
       {
         id: `${chartKey}-x-slider`, type: "slider", xAxisIndex: 0, filterMode: "none",
         height: 16, bottom: 12, borderColor: border, backgroundColor: surface,
-        fillerColor: colors.all.replace("rgb(", "rgb(").replace(")", " / .16)"),
+        fillerColor: colors.all.replace(/\)$/, " / .16)"),
         handleStyle: { color: colors.all, borderColor: colors.all },
         textStyle: { color: dim, fontSize: 9 }, showDetail: false, ...range(xRange),
       },
@@ -375,7 +375,7 @@ export class StatisticsCharts {
       {
         id: `${chartKey}-y-slider`, type: "slider", yAxisIndex: 0, orient: "vertical", filterMode: "none",
         width: 14, right: 8, top: 38, bottom: 66, borderColor: border, backgroundColor: surface,
-        fillerColor: colors.all.replace("rgb(", "rgb(").replace(")", " / .12)"),
+        fillerColor: colors.all.replace(/\)$/, " / .12)"),
         handleStyle: { color: colors.all, borderColor: colors.all },
         textStyle: { color: dim, fontSize: 9 }, showDetail: false, ...range(yRange),
       },
diff --git a/tests/help-window.test.mjs b/tests/help-window.test.mjs
index c828dd8..ba330c4 100644
--- a/tests/help-window.test.mjs
+++ b/tests/help-window.test.mjs
@@ -32,6 +32,8 @@ test("help is available from the utility menu and F1 through one window lifecycl
 test("help page is routed independently and receives language and theme updates", () => {
   assert.match(entrySource, /page\.get\("help"\) === "1"/);
   assert.match(entrySource, /import\("\.\/help-window"\)/);
+  assert.match(entrySource, /appRoot\.replaceChildren\(fatal\)/);
+  assert.doesNotMatch(entrySource, /appRoot\.innerHTML/);
   assert.match(windowSource, /listen<HelpWindowPayload>\("help:state"/);
   assert.match(windowSource, /setLanguagePreference\(payload\.language\)/);
   assert.match(windowSource, /document\.documentElement\.dataset\.theme = payload\.theme/);
@@ -89,7 +91,8 @@ test("all seven interface locales provide a complete localized manual", () => {
   assert.match(localizedContentSource, /if \(locale === "zh-CN"\)/);
   assert.match(localizedContentSource, /V0\.5\.5/g);
   assert.match(windowSource, /data-help-locale="\$\{getResolvedLocale\(\)\}"/);
-  assert.match(windowSource, /isLanguagePreference\(requestedLanguage\)/);
+  assert.match(windowSource, /queryLanguagePreference\(new URLSearchParams/);
+  assert.doesNotMatch(windowSource, /isLanguagePreference\(requestedLanguage\)/);
   assert.match(windowSource, /this\.catalog = getHelpCatalog\(getResolvedLocale\(\)\)/);
   assert.match(windowSource, /if \(getResolvedLocale\(\) !== previousLocale\)/);
   assert.match(windowSource, /this\.render\(\)/);
diff --git a/tests/image-statistics.test.mjs b/tests/image-statistics.test.mjs
index 61f0d43..2dfc89f 100644
--- a/tests/image-statistics.test.mjs
+++ b/tests/image-statistics.test.mjs
@@ -97,6 +97,7 @@ test("statistics view supports three presentations, independent curves, two-axis
   assert.match(chartSource, /zoomOnMouseWheel:\s*"ctrl"/);
   assert.match(chartSource, /id: `\$\{chartKey\}-y-inside`[\s\S]*?zoomOnMouseWheel:\s*"shift"/);
   assert.match(chartSource, /orient: "vertical"/);
+  assert.doesNotMatch(chartSource, /\.replace\("rgb\(", "rgb\("\)/);
   assert.match(chartSource, /backgroundColor: "transparent"/);
   assert.match(panelSource, /data-stat-group-chart=/);
   assert.match(panelSource, /data-stat-y-reset=/);