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Element “script” must not have attribute “async” unless attribute “src” is also specified or unless attribute “type” is specified with value “module”.

The async and defer boolean attributes of the <script> element control how an external script should be executed once it has been downloaded. The async attribute makes sense when an external script (defined with the src attribute) is loaded, or when defining a script of type module:

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

<script type="module">
  /* JavaScript module code here */

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