allow document level hydration (#5547).

src/runtime/internal/dom.ts

@@ -5,8 +5,10 @@ export function append(target: Node, node: Node) {
 }
 
 export function insert(target: Node, node: Node, anchor?: Node) {
+	if (target != document) {
 		target.insertBefore(node, anchor || null);
 	}
+}
 
 export function detach(node: Node) {
 	node.parentNode.removeChild(node);