add sanity check mutation observer

Author: JoviDeCroock

demo/src/entry-client.jsx

@@ -1,4 +1,19 @@
-// import { hydrate } from 'preact';
-// import { App } from './App';
+import { hydrate } from 'preact';
+import { App } from './App';
 
-// hydrate(<App />, document);
+const config = { attributes: true, childList: true, subtree: true };
+const mut = new MutationObserver((mutationList, observer) => {
+	for (const mutation of mutationList) {
+		if (mutation.type === 'childList') {
+			console.log('A child node has been added or removed.', mutation);
+		} else if (mutation.type === 'attributes') {
+			console.log(
+				`The ${mutation.attributeName} attribute was modified.`,
+				mutation
+			);
+		}
+	}
+});
+mut.observe(document, config);
+
+hydrate(<App />, document);