minor fixes

2-ui/5-loading/01-onload-ondomcontentloaded/article.md

@@ -185,6 +185,26 @@ window.onbeforeunload = function() {
 
 The behavior was changed, because some webmasters abused this event handler by showing misleading and annoying messages. So right now old browsers still may show it as a message, but aside of that -- there's no way to customize the message shown to the user.
 
+````warn header="The `event.preventDefault()` doesn't work from a `beforeunload` handler"
+That may sound weird, but most browsers ignore `event.preventDefault()`.
+
+Which means, following code may not work:
+```js run
+window.addEventListener("beforeunload", (event) => {
+  // doesn't work, so this event handler doesn't do anything
+	event.preventDefault();
+});
+```
+
+Instead, in such handlers one should set `event.returnValue` to a string to get the result similar to the code above:
+```js run
+window.addEventListener("beforeunload", (event) => {
+  // same as returning from window.onbeforeunload
+	event.returnValue = "There are unsaved changes. Leave now?";
+});
+```
+````
+
 ## readyState
 
 What happens if we set the `DOMContentLoaded` handler after the document is loaded?