HTML async Attribute: Reference and Examples

The async attribute is a small instruction attached to an HTML element. Its job is to download and execute a classic script as soon as it is ready. It is easier to remember when you can see it in a real piece of markup.

What async is used for

Download and execute a classic script as soon as it is ready. The attribute appears inside an element’s opening tag. Depending on its definition, it may take a specific string, a number, an enumerated keyword, or be a boolean attribute.

The example below uses it in a script context. Other elements may support it too, or may give the same attribute a different meaning.

Example you can try

<head>
    <script src="/js/app.js" async></script>
</head>

How the example works

Look for async in the snippet. Here the example shows its presence without a string value. This is a concrete input the browser can interpret; it is not just text shown to the reader.

Async classic scripts may execute in a different order from their appearance in the document.

Values and important behavior

Use the value format shown by the HTML definition. Do not assume that a value accepted in one attribute also works in another. When a browser ignores an invalid value, inspect the element and correct the markup.

This detail matters most when the page receives user input, navigates to another document, or loads an external resource. In those cases, check the browser behavior and any server-side requirements separately.

Small practice task

Copy the code and change async to another sensible value, or remove it entirely if it is optional. Test the difference in the browser and inspect any submitted value or network request.

Common Beginner Mistakes

• Async classic scripts may execute in a different order from their appearance in the document.
• Using async on an element that does not support it.
• Believing that an HTML hint or client-side check replaces real server validation or a security policy.

Key Takeaways

• async is used to download and execute a classic script as soon as it is ready.
• Async classic scripts may execute in a different order from their appearance in the document.
• Choose valid values and test the result instead of guessing what the browser will do.