HTML Validation

Bad value “X” for attribute “async” on element “script”.

About This HTML Issue

The async attribute is boolean: the presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value. As a boolean attribute, it does not need to be passed any value such as true or 1 to activate the async property.

For classic scripts, if the async attribute is present, then the classic script will be fetched in parallel to parsing and evaluated as soon as it is available.

For module scripts, if the async attribute is present then the scripts and all their dependencies will be executed in the defer queue, therefore they will get fetched in parallel to parsing and evaluated as soon as they are available.

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

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

Learn more:

→ MDN: Script attributes

Last reviewed: March 14, 2023

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Related W3C Validator Issues (74)

Bad value “” for attribute “src” on element “script”: Must be non-empty.

Ensure the src attribute on the script element is non-empty and points to a valid resource.

The src attribute in a script element specifies the URL of an external script file. An empty src attribute is invalid because it tells the browser to fetch a resource from a URL that is not provided, leading to loading errors. Instead, ensure that the src attribute contains a valid file path or URL to an existing script file. If the script content is meant to be inline, you should omit the src attribute altogether and include the script content directly within the script element.

Example of a Valid External Script

Here is a valid example of a script element with a non-empty src attribute:

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Valid Script Example</title>
</head>
<body>
  <script src="path/to/script.js"></script>
</body>
</html>

Example of a Valid Inline Script

If the script is to be written inline, exclude the src attribute and write the JavaScript code directly within the script tags:

<!DOCTYPE html>
<html lang="en">
<head>
  <title>Inline Script Example</title>
</head>
<body>
  <script>
    console.log('This is an inline script.');
  </script>
</body>
</html>

Troubleshooting

Double-check the script’s file path:

Ensure the file path you provide in the src is correct relative to the HTML file.
Make sure the script file exists in the location specified.
If using a network URL, verify that the URL is correct and accessible.

Bad value X for attribute “src” on element “script”: Illegal character in path segment: space is not allowed.

The issue arises from the space character in the src attribute value of the script element. In URLs, spaces are not allowed and should be properly encoded to avoid validation errors.

Fix

Replace spaces with %20, which is the URL-encoded representation of a space.

Example

Before:

<script src="https://example.com/media assets/app.js"></script>

After:

<script src="https://example.com/media%20assets/app.js"></script>

Explanation

In this example, the space between “media” and “assets” in the URL is replaced with %20. This change ensures that the URL conforms to standards and is correctly processed by browsers and servers. Spaces and other special characters in URLs must be encoded to ensure proper formatting and accessibility.

Bad value X for attribute “src” on element “script”: Illegal character in query: “"” is not allowed.

There is an illegal double quote character (") at the end of the src attribute value in your <script> tag, which causes the attribute to be invalid.

Attribute values must not include unescaped or stray quote characters (" or ') inside them, as this breaks attribute parsing and results in invalid HTML. The src attribute for a <script> tag should contain a properly encoded URL without any stray quotes or illegal characters. In your case, a double quote has accidentally been included before the closing quote of the attribute.

Correct usage for a <script> tag with the async attribute is:

<script src="https://example.com/js/jquery-3.6.0.min.js?ver=6.8.2" async></script>

Incorrect example with the error (shows the issue):

<script src="https://example.com/js/jquery-3.6.0.min.js?ver=6.8.2" async""></script>

Make sure there are no stray characters in your attribute values and that boolean attributes like async do not have values—it should simply be present, as in async, not async"" or async="async".

If you need a full, minimal HTML document to validate, use:

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Valid Script Tag Example</title>
  </head>
  <body>
    <script src="https://example.com/js/jquery-3.6.0.min.js?ver=6.8.2" async></script>
  </body>
</html>

Double-check your HTML source code to ensure there are no accidental typos or misplaced quote marks in your tag attributes.

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

A “script” element with a “defer” attribute must not have a “type” attribute with the value “module”.

The combination of type="module" and defer is not allowed. The type="module" attribute itself implies that the script should be executed in a deferred way, hence using the defer attribute is unnecessary and invalid.

Steps to Fix the Issue:

Remove the defer Attribute: When you use type="module", you should not include the defer attribute since module scripts defer automatically.

Incorrect Code:

<script type="module" defer src="example.js"></script>

Corrected Code:

<script type="module" src="example.js"></script>

A “script” element with a “src” attribute must not have a “type” attribute whose value is anything other than the empty string, a JavaScript MIME type, or “module”.

The <script> tag allows authors to include dynamic scripts and data blocks in their documents. When the src is present, this tag accepts a type attribute which must be either:

an empty string
text/javascript (that’s the default, so it can be omitted)
module

Examples:

<!-- This is valid, without a type it defaults to JavaScript -->

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

<!-- This is valid, but will warn that it can be omitted -->

<script type="text/javascript" src="app.js"></script>

<!-- An empty attribute is valid, but will warn that it can be omitted -->

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

<!-- The module keyword is also valid as a type -->

<script type="module" src="app.js"></script>

<!-- Any other type is invalid -->

<script type="wrong" src="app.js"></script>
<script type="text/html" src="app.js"></script>
<script type="image/jpeg" src="app.js"></script>

Bad value (true|false) for attribute “itemscope” on element “html”.

The itemscope attribute is a boolean attribute in HTML5, which means it does not take any values. Adding any value (such as true or false) will cause an error. When using boolean attributes, they should either be present or absent. If an attribute like itemscope is present, it is considered true.

Here’s how to correct the error:

Incorrect Usage:

<div itemscope="true">

Correct Usage:

<div itemscope>

Explanation:

Simply including the itemscope attribute without any value is the correct way to use it.
If you don’t want to use the itemscope attribute, just remove it from the tag.

Bad value “callto:X” for attribute “href” on element “a”: Illegal character in scheme data: space is not allowed.

Spaces are not permitted in the href value for phone links; the phone number must be a continuous string without spaces or slashes.

The href attribute of an anchor (<a>) element defines the link’s destination. For phone numbers, the proper URI scheme is tel:, not callto:. According to the HTML standard and the WHATWG Living Standard, the phone number should contain only digits and may use plus (+) or hyphen (-) characters for formatting, but it should not include spaces or slashes.

Incorrect HTML:

<a href="callto:07142/ 12 34 5">Call us</a>

Correct HTML:

<a href="tel:0714212345">Call us</a>

With country code and optional formatting:

<a href="tel:+49714212345">Call us</a>

For best compatibility and validation, always use the tel: scheme and ensure the phone number string contains only allowed characters.

Bad value “contact” for attribute “autocomplete” on element “input”: The string “contact” is not a valid autofill field name.

Bad value “dialog” for attribute “role” on element “dialog”.

The dialog element does not require or permit a role="dialog" attribute according to HTML standards.

The <dialog> element has an implicit ARIA role of dialog, so adding role="dialog" is redundant and not valid per the specification. Instead, simply use the <dialog> element without an explicit role attribute.

Details:

According to the WHATWG HTML standard and ARIA specification, native <dialog> elements automatically have the correct role. Adding role="dialog" can cause HTML validation errors, as the validator interprets this as a misuse or redundancy.

Correct usage:

<dialog>
  <p>This is a dialog box.</p>
  <button>Close</button>
</dialog>

Incorrect usage (causes validation error):

<dialog role="dialog">
  <p>This is a dialog box.</p>
  <button>Close</button>
</dialog>

Removing the role="dialog" attribute resolves the W3C validation issue while maintaining accessibility.

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