The `async` attribute is a boolean that sets whether the file should be executed asynchronously or not.

```html
<script async="true" src="javascript.js"></script>
```

It should be:

```html
<script async src="javascript.js"></script>
```

# How do I fix this ?

This attribute should either be present or not.

# Resources

* [Script element](https://developer.mozilla.org/en/docs/Web/API/HTMLScriptElement)
* [MDN script](https://developer.mozilla.org/en/docs/Web/HTML/Element/script)