diff --git a/crates/obscura-js/js/bootstrap.js b/crates/obscura-js/js/bootstrap.js
index eae6b38..f58c78c 100644
--- a/crates/obscura-js/js/bootstrap.js
+++ b/crates/obscura-js/js/bootstrap.js
@@ -9004,6 +9004,17 @@ Element.prototype.attachShadow = function attachShadow(opts) {
   }
   const host = this;
   const children = [];
+  const assertInsertable = (node, operation) => {
+    const createsComposedCycle = node instanceof ShadowRoot
+      || node === host
+      || !!(node?.contains && node.contains(host));
+    if (createsComposedCycle) {
+      throw new DOMException(
+        `Failed to execute '${operation}' on 'Node': The new child would contain the parent.`,
+        "HierarchyRequestError"
+      );
+    }
+  };
   const shadow = {
     mode: opts.mode,
     host: host,
@@ -9022,44 +9033,72 @@ Element.prototype.attachShadow = function attachShadow(opts) {
     get firstElementChild() { return children.find(c => c.nodeType === 1) || null; },
     get children() { return children.filter(c => c.nodeType === 1); },
     appendChild(c) {
-      if (c) {
-        if (c._shadowParent) c._shadowParent.removeChild(c);
-        else if (c.parentNode) c.parentNode.removeChild(c);
-        children.push(c);
-        c._shadowParent = shadow;
+      if (!c) return c;
+      assertInsertable(c, "appendChild");
+      if (c instanceof DocumentFragment) {
+        for (const child of Array.from(c.childNodes)) shadow.appendChild(child);
+        return c;
       }
+      if (c._shadowParent) c._shadowParent.removeChild(c);
+      else if (c.parentNode) c.parentNode.removeChild(c);
+      children.push(c);
+      c._shadowParent = shadow;
       return c;
     },
     insertBefore(n, ref) {
       if (!n) return n;
       if (!ref) { shadow.appendChild(n); return n; }
-      const idx = children.indexOf(ref);
-      if (idx >= 0) {
-        if (n._shadowParent) n._shadowParent.removeChild(n);
-        else if (n.parentNode) n.parentNode.removeChild(n);
-        children.splice(idx, 0, n);
-        n._shadowParent = shadow;
+      if (!children.includes(ref)) {
+        throw new DOMException(
+          "Failed to execute 'insertBefore' on 'Node': The reference node is not a child of this node.",
+          "NotFoundError"
+        );
       }
-      else shadow.appendChild(n);
+      if (n === ref) return n;
+      assertInsertable(n, "insertBefore");
+      if (n instanceof DocumentFragment) {
+        for (const child of Array.from(n.childNodes)) shadow.insertBefore(child, ref);
+        return n;
+      }
+      if (n._shadowParent) n._shadowParent.removeChild(n);
+      else if (n.parentNode) n.parentNode.removeChild(n);
+      const idx = children.indexOf(ref);
+      children.splice(idx, 0, n);
+      n._shadowParent = shadow;
       return n;
     },
     removeChild(c) {
       const idx = children.indexOf(c);
-      if (idx >= 0) {
-        children.splice(idx, 1);
-        if (c._shadowParent === shadow) c._shadowParent = null;
+      if (idx < 0) {
+        throw new DOMException(
+          "Failed to execute 'removeChild' on 'Node': The node to be removed is not a child of this node.",
+          "NotFoundError"
+        );
       }
+      children.splice(idx, 1);
+      if (c._shadowParent === shadow) c._shadowParent = null;
       return c;
     },
     replaceChild(n, o) {
-      const idx = children.indexOf(o);
-      if (idx >= 0) {
-        if (n._shadowParent) n._shadowParent.removeChild(n);
-        else if (n.parentNode) n.parentNode.removeChild(n);
-        children[idx] = n;
-        n._shadowParent = shadow;
-        if (o._shadowParent === shadow) o._shadowParent = null;
+      if (!children.includes(o)) {
+        throw new DOMException(
+          "Failed to execute 'replaceChild' on 'Node': The node to be replaced is not a child of this node.",
+          "NotFoundError"
+        );
       }
+      if (n === o) return o;
+      assertInsertable(n, "replaceChild");
+      if (n instanceof DocumentFragment) {
+        for (const child of Array.from(n.childNodes)) shadow.insertBefore(child, o);
+        shadow.removeChild(o);
+        return o;
+      }
+      if (n._shadowParent) n._shadowParent.removeChild(n);
+      else if (n.parentNode) n.parentNode.removeChild(n);
+      const idx = children.indexOf(o);
+      children[idx] = n;
+      n._shadowParent = shadow;
+      if (o._shadowParent === shadow) o._shadowParent = null;
       return o;
     },
     querySelector(s) {
diff --git a/crates/obscura-js/src/runtime.rs b/crates/obscura-js/src/runtime.rs
index a966ebd..f5ea935 100644
--- a/crates/obscura-js/src/runtime.rs
+++ b/crates/obscura-js/src/runtime.rs
@@ -1993,7 +1993,50 @@ mod tests {
                     inside.getRootNode() === root,
                     root.textContent
                 ];
-                return [inserted, removed, moved, parsed];
+
+                const a = document.createElement("a");
+                const b = document.createElement("b");
+                const c = document.createElement("i");
+                root.replaceChildren(a, b, c);
+                root.insertBefore(a, c);
+                const movedWithin = Array.from(root.children, el => el.localName);
+
+                const fragment = document.createDocumentFragment();
+                const x = document.createElement("x-one");
+                const y = document.createElement("x-two");
+                fragment.append(x, y);
+                root.insertBefore(fragment, c);
+                const flattened = [
+                    Array.from(root.children, el => el.localName),
+                    fragment.childNodes.length,
+                    x.parentNode === root,
+                    y.parentNode === root
+                ];
+
+                root.replaceChild(b, c);
+                const replaced = [
+                    Array.from(root.children, el => el.localName),
+                    c.parentNode === null,
+                    b.parentNode === root
+                ];
+
+                const detached = document.createElement("detached-node");
+                const errors = [];
+                for (const operation of [
+                    () => root.insertBefore(detached, c),
+                    () => root.removeChild(c),
+                    () => root.replaceChild(detached, c),
+                    () => root.appendChild(root),
+                    () => root.appendChild(host)
+                ]) {
+                    try {
+                        operation();
+                        errors.push("none");
+                    } catch (error) {
+                        errors.push(error.name);
+                    }
+                }
+                return [inserted, removed, moved, parsed, movedWithin, flattened, replaced, errors];
                 "#,
             )
             .unwrap();
@@ -2016,7 +2059,17 @@ mod tests {
                 ],
                 [true, true, true],
                 [true, true, true],
-                [true, true, "inside"]
+                [true, true, "inside"],
+                ["b", "a", "i"],
+                [["b", "a", "x-one", "x-two", "i"], 0, true, true],
+                [["a", "x-one", "x-two", "b"], true, true],
+                [
+                    "NotFoundError",
+                    "NotFoundError",
+                    "NotFoundError",
+                    "HierarchyRequestError",
+                    "HierarchyRequestError"
+                ]
             ])
         );
     }