HTML Validation

Attribute “X” not allowed on element “Y” at this point.

About This HTML Issue

An invalid attribute has been found on an element. Check the affected tag to ensure attributes are well-formed, and if they are you can consider using custom data attributes.

Learn more:

→ HTML Spec: custom data attributes

Last reviewed: September 02, 2024

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Related W3C Validator Issues (11)

Attribute “<” not allowed on element “X” at this point.

Check the syntax of the affected tag, it’s probably malformed and a < character inside has been interpreted as an attribute.

For example, this code might cause this issue:

<!-- Malformed img tag -->

<img src="photo.jpg" alt="smiling cat" < />

<!-- Fixed img tag -->

<img src="photo.jpg" alt="smiling cat" />

Attribute “xmlns:dt” not allowed here.

The xmlns:dt attribute is not permitted on standard HTML elements according to the HTML specification.

HTML5 does not use XML namespaces like xmlns:dt, which are only valid in certain XML vocabularies such as XHTML or when embedding MathML or SVG. In typical HTML, attributes with xmlns or any custom XML namespace prefixes are invalid and cause validation errors.

To fix this issue, simply remove the xmlns:dt attribute from your HTML tags.
If you are using a data attribute or a custom attribute, you can use data-* attributes instead, which are allowed in HTML5.

Incorrect usage with xmlns:dt:

<div xmlns:dt="urn:schemas-microsoft-com:datatypes">
  Content here
</div>

Correct usage—attribute removed:

<div>
  Content here
</div>

If you need to store custom data, use data-* attributes:

<div data-dt="urn:schemas-microsoft-com:datatypes">
  Content here
</div>

Avoid using XML namespaces in HTML5 documents to ensure your code is standards-compliant.

Attribute “displaytext” not allowed on element “span” at this point.

Attribute “height” not allowed on element “table” at this point.

Attribute “st_title” not allowed on element “span” at this point.

Attribute “st_url” not allowed on element “span” at this point.

Attribute with the local name “xmlns:serif” is not serializable as XML 1.0

Attribute “X” is not serializable as XML 1.0.

Attribute “xmlns:m” not allowed here.

The xmlns:m attribute is not permitted on the html element in HTML5 documents.

HTML5 allows only certain attributes on the html element, specifically lang and dir (and the standard xmlns in XHTML serialization contexts), but not custom XML namespaces like xmlns:m. The xmlns:m attribute is typically used in XML-based documents (such as MathML or Office markup) and is not valid in standard HTML5 syntax.

To resolve the error, remove the xmlns:m attribute from the html element. If you need to use namespace-prefixed elements or attributes (such as for Office documents or MathML), you should use XHTML serialization (served as application/xhtml+xml) or rework your markup to be fully compatible with HTML5 without custom namespaces.

Incorrect usage:

<!DOCTYPE html>
<html xmlns:m="http://schemas.microsoft.com/office/2004/12/omml">
  <head>
    <title>Invalid Namespace Example</title>
  </head>
  <body>
<!-- Content here -->

  </body>
</html>

Correct HTML5 usage:

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Valid HTML Example</title>
  </head>
  <body>
<!-- Content here -->

  </body>
</html>

If you require namespaces for specific elements (like MathML), use them directly without declaring a namespace on the html element, or switch to XHTML if your application truly requires namespace declarations. For most web applications, sticking to standard HTML5 elements and attributes ensures maximal compatibility and validator compliance.

Attribute “xmlns:v” not allowed here.

The xmlns:v attribute is not valid on the html element in HTML5.

HTML5 does not support custom XML namespaces like xmlns:v, which was used in old versions of Internet Explorer for VML (Vector Markup Language). The xmlns:v attribute is unnecessary and should be removed to make your HTML valid.

Example before (invalid HTML):

<!DOCTYPE html>
<html xmlns:v="urn:schemas-microsoft-com:vml">
  <head>
    <title>VML Example</title>
  </head>
  <body>
<!-- content -->

  </body>
</html>

Example after (valid HTML):

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>VML Example</title>
  </head>
  <body>
<!-- content -->

  </body>
</html>

For vector graphics in modern HTML, use the svg element instead of legacy VML.

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