HTML Guide
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.
Related W3C validator issues
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.
The attribute displayText is not allowed on <span> elements.
This issue is commonly caused by an old integration of ShareThis via Drupal or other CMS - the old code used invalid attributes like displayText, st_url and st_title which were later changed to HTML5 custom data attributes.
The <table> element does not accept a height attribute. Use CSS instead.
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" />The attribute st_title is not allowed on <span> elements.
This issue is commonly caused by an old integration of ShareThis via Drupal or other CMS - the old code used invalid attributes like displayText, st_url and st_title which were later changed to HTML5 custom data attributes.
The attribute st_url is not allowed on <span> elements.
This issue is commonly caused by an old integration of ShareThis via Drupal or other CMS - the old code used invalid attributes like displayText, st_url and st_title which were later changed to HTML5 custom data attributes.
Using the xmlns:dt attribute in the <html> tag is invalid in HTML5 and triggers validation errors because custom XML namespaces are not supported in HTML.
HTML5 does not support arbitrary XML namespaces, as used in XHTML or other XML-based vocabularies. The attribute xmlns:dt is specific to XML serialization and not serializable as per XML 1.0 rules when used in HTML5 documents, which no longer use the XML namespace mechanism. The only allowed use of xmlns in HTML is for SVG and MathML in embedded contexts, under specific interoperability rules.
To resolve this, remove the xmlns:dt attribute entirely from your document and use a standard HTML <html> declaration.
Incorrect (causes validation error):
<html xmlns:dt="uuid:C2F41010-65B3-11d1-A29F-00AA00C14882">
</html>Correct (W3C-compliant HTML5):
<!DOCTYPE html>
<html lang="en">
<head>
<title>No XMLNS Namespace</title>
</head>
<body>
<!-- Your content here -->
</body>
</html>Remove XML-specific attributes from HTML documents to ensure compatibility and compliance with modern HTML standards.
The xmlns:m attribute is an XML namespace declaration and not valid on the html element in HTML documents.
HTML5 does not support custom XML namespaces; they are only used in XML-based languages such as XHTML. The global xmlns:* attributes like xmlns:m are not allowed on the <html> element in HTML documents that use the text/html MIME type. This causes serialization and validation errors, especially with W3C HTML validation.
To fix this, remove the xmlns:m attribute from the <html> tag. If you need to use a namespace, your document should be XHTML and served as application/xhtml+xml, not as regular HTML.
Incorrect HTML (causes validation error):
<html xmlns:m="http://schemas.microsoft.com/office/2004/12/omml">
<head>
<title>Invalid Namespace Example</title>
</head>
<body>
<!-- Content -->
</body>
</html>Correct HTML (remove the namespace declaration):
<!DOCTYPE html>
<html lang="en">
<head>
<title>Valid HTML Example</title>
</head>
<body>
<!-- Content -->
</body>
</html>If you require usage of MathML, SVG, or other XML vocabularies, HTML5 already supports them natively without explicit namespace declarations. For Microsoft Office Markup Language (OMML), use proper XML/XHTML serialization and the correct MIME type if absolutely necessary. For typical web content, avoid custom XML namespaces in HTML5.
The attribute xmlns:serif is not valid. Check this guide for more information on this issue.
Remove the xmlns:svg attribute from the <svg> element, leaving only the valid xmlns attribute.
The xmlns attribute defines the XML namespace for the SVG and is correctly set to http://www.w3.org/2000/svg. The xmlns:svg attribute attempts to declare a prefixed namespace, which is unnecessary and invalid in both HTML and SVG served as XML (or embedded in HTML). The W3C validator flags this because HTML5 does not allow arbitrary XML namespace declarations beyond the standard SVG namespace; such prefixes are not used in HTML serialization.
Incorrect usage:
<svg xmlns="http://www.w3.org/2000/svg" xmlns:svg="http://www.w3.org/2000/svg">
<!-- SVG content -->
</svg>Correct usage:
<svg xmlns="http://www.w3.org/2000/svg">
<!-- SVG content -->
</svg>Only xmlns="http://www.w3.org/2000/svg" is required for inline SVG in HTML.