diff --git a/README.md b/README.md
index aefaee4..2a1a730 100644
--- a/README.md
+++ b/README.md
@@ -6,6 +6,92 @@
   }
 </style>
 <!-- Navigation behavior update marker -->
+<script>
+  (() => {
+    const storageKey = "zensical-nav-state-v1";
+    let lastSignature = "";
+
+    const getToggles = () =>
+      Array.from(document.querySelectorAll("input.md-nav__toggle.md-toggle[id]"));
+
+    const getPrimaryList = () =>
+      document.querySelector("nav.md-nav--primary > ul.md-nav__list");
+
+    const applyDefaultTopLevelState = (toggles) => {
+      const primaryList = getPrimaryList();
+      if (!primaryList) {
+        return;
+      }
+
+      const topLevelToggles = Array.from(
+        primaryList.querySelectorAll(":scope > li > input.md-nav__toggle.md-toggle[id]")
+      );
+      const topLevelIds = new Set(topLevelToggles.map((toggle) => toggle.id));
+
+      for (const toggle of toggles) {
+        toggle.checked = topLevelIds.has(toggle.id);
+      }
+    };
+
+    const restoreState = (toggles) => {
+      const raw = localStorage.getItem(storageKey);
+      if (!raw) {
+        applyDefaultTopLevelState(toggles);
+        return;
+      }
+
+      try {
+        const state = JSON.parse(raw);
+        for (const toggle of toggles) {
+          if (Object.prototype.hasOwnProperty.call(state, toggle.id)) {
+            toggle.checked = !!state[toggle.id];
+          }
+        }
+      } catch {
+        applyDefaultTopLevelState(toggles);
+      }
+    };
+
+    const persistState = (toggles) => {
+      const state = {};
+      for (const toggle of toggles) {
+        state[toggle.id] = !!toggle.checked;
+      }
+      localStorage.setItem(storageKey, JSON.stringify(state));
+    };
+
+    const initialize = () => {
+      const toggles = getToggles();
+      if (toggles.length === 0) {
+        return;
+      }
+
+      const signature = toggles.map((toggle) => toggle.id).join("|");
+      if (signature === lastSignature) {
+        return;
+      }
+
+      lastSignature = signature;
+      restoreState(toggles);
+      for (const toggle of toggles) {
+        if (toggle.dataset.navStateBound === "true") {
+          continue;
+        }
+
+        toggle.dataset.navStateBound = "true";
+        toggle.addEventListener("change", () => persistState(getToggles()));
+      }
+    };
+
+    if (document.readyState === "loading") {
+      document.addEventListener("DOMContentLoaded", initialize, { once: true });
+    } else {
+      initialize();
+    }
+
+    setInterval(initialize, 500);
+  })();
+</script>
 
 {{ DESCRIPTION }}