minor fixes

2-ui/5-loading/02-script-async-defer/article.md

@@ -133,6 +133,10 @@ Async scripts are great when we integrate an independent third-party script into
 <script async src="https://google-analytics.com/analytics.js"></script>
 ```
 
+```smart header="The `async` attribute is only for external scripts"
+Just like `defer`, the `async` attribute is ignored if the `<script>` tag has no `src`.
+```
+
 ## Dynamic scripts
 
 There's one more important way of adding a script to the page.