HTML Checking for Large Sites

Rocket Validator integrates the W3C Validator HTML checker into an automated web crawler.

The defer and async boolean attributes of the <script> element control how an external script should be executed once it has been downloaded. These attributes only make sense when referring to external scripts, so a src attribute must also be present to specify the location of the script.

Example:

<script defer src="app.js"></script>

If your script is not external, and is inlined within the HTML document, then you should remove the defer attribute, like in this example:

<script>
  console.log("hello");
</script>

Learn more:

Related W3C validator issues