diff --git a/hemx-js/runtime/hemx.js b/hemx-js/runtime/hemx.js
index 7c31dbd..fcee9f0 100644
--- a/hemx-js/runtime/hemx.js
+++ b/hemx-js/runtime/hemx.js
@@ -791,11 +791,20 @@
 
   function start() {
     roots().forEach((root) => {
-      bootstrapState(root);
-      bindRoot(root);
-      bindSse(root);
+      try {
+        bootstrapState(root);
+        bindRoot(root);
+        bindSse(root);
+      } catch (error) {
+        emit(root, "hemx:error", String(error));
+      }
     });
-    history.replaceState(history.state || { hemx: true }, "", location.href);
+    try {
+      history.replaceState(history.state || { hemx: true }, "", location.href);
+    } catch (error) {
+      const root = roots()[0];
+      if (root) emit(root, "hemx:history-error", String(error));
+    }
   }
 
   addEventListener("popstate", () => {
@@ -803,8 +812,8 @@
     if (root) navigateUrl(location.href, root, "none").catch((error) => emit(root, "hemx:error", String(error)));
   });
 
+  window.hemx = Object.freeze({ runtimeAbiVersion, roots, rootOf, applyHtml, applyBatch, decodeBatch, atomValue, decodeAtomState });
+
   if (document.readyState === "loading") document.addEventListener("DOMContentLoaded", start);
   else start();
-
-  window.hemx = Object.freeze({ runtimeAbiVersion, roots, rootOf, applyHtml, applyBatch, decodeBatch, atomValue, decodeAtomState });
 })();
diff --git a/hemx-js/tests/runtime.rs b/hemx-js/tests/runtime.rs
index 47dee5c..5ef2a9d 100644
--- a/hemx-js/tests/runtime.rs
+++ b/hemx-js/tests/runtime.rs
@@ -1,3 +1,19 @@
+#[test]
+fn runtime_exposes_debug_api_before_startup_side_effects() {
+    // req: runtime/002
+    let source = hemx_js::RUNTIME_JS;
+
+    let api = source
+        .find("window.hemx = Object.freeze")
+        .expect("runtime exposes browser API");
+    let start = source
+        .find("if (document.readyState === \"loading\") document.addEventListener(\"DOMContentLoaded\", start)")
+        .expect("runtime starts after declaration");
+    assert!(api < start);
+    assert!(source.contains("try {\n        bootstrapState(root);"));
+    assert!(source.contains("emit(root, \"hemx:error\", String(error));"));
+}
+
 #[test]
 fn runtime_posts_urlencoded_forms_by_default() {
     let source = hemx_js::RUNTIME_JS;