HTML Guide

Bad value “http://www.w3.org/TR/REC-html40” for the attribute “xmlns” (only “http://www.w3.org/1999/xhtml” permitted here).

xmlns

Only http://www.w3.org/1999/xhtml is allowed as the value for the xmlns attribute in HTML5 documents.

The xmlns attribute specifies the XML namespace of an element and is mainly used with XHTML documents. For HTML5, the correct value is http://www.w3.org/1999/xhtml, and typically you don’t need to include the xmlns attribute at all unless you are serving your document as application/xhtml+xml (XHTML). Using the http://www.w3.org/TR/REC-html40 value is invalid and will trigger validator errors.

Correct code for a standard HTML5 document:

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Valid HTML5 Example</title>
  </head>
  <body>
    <!-- Page content -->
  </body>
</html>

If you are using XHTML and need the namespace:

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang="en">
  <head>
    <title>Valid XHTML Example</title>
  </head>
  <body>
    <!-- Page content -->
  </body>
</html>

For HTML5, simply remove the xmlns attribute or, if serving as XHTML, ensure the value is exactly http://www.w3.org/1999/xhtml.

Related W3C validator issues

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

xmlns

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.

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

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.

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

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

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.

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

The xmlns:v attribute is not valid in HTML5 and should be removed to ensure compliance with W3C standards.

The xmlns:v namespace attribute was historically used for VML (Vector Markup Language), mainly in legacy support for Internet Explorer. Modern HTML, particularly the living standard and HTML5, does not permit namespace declarations using xmlns attributes like xmlns:v. These are only valid in XML-based serializations, such as XHTML. Including xmlns:v in a standard HTML5 document results in validation errors because HTML does not support custom namespaces.

Incorrect HTML:

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

Corrected HTML:

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>VML Namespace Removed</title>
  </head>
  <body>
    <!-- Content -->
  </body>
</html>

Remove the xmlns:v attribute entirely to resolve the validator warning and ensure your HTML meets current standards. If you are not using VML, no further changes are necessary. If you require vector graphics, use SVG instead, which is fully supported in HTML5.

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

xmlns

Using the attribute xmlns:w="urn:schemas-microsoft-com:office:word" in HTML is invalid because custom XML namespace declarations are not allowed in HTML5.

HTML5 does not support custom namespaces like xmlns:w, which are used in XML-based formats such as XHTML or Office documents, but not in regular HTML. The HTML parser ignores unknown attributes and does not treat them as namespaces, which can result in validation errors. Only predefined namespaces allowed by the HTML specification (such as the default for SVG or MathML) are supported via the appropriate embedding elements.

Correct HTML Example Without Custom Namespace:

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>No Custom Namespace Example</title>
  </head>
  <body>
    <p>Custom XML namespaces like <code>xmlns:w</code> are not valid in HTML5.</p>
  </body>
</html>

Incorrect Example That Causes the Error:

<html xmlns:w="urn:schemas-microsoft-com:office:word">
  <!-- ... -->
</html>

To fix the error, remove the xmlns:w attribute from your HTML. If you need to use Office-specific features or XML namespaces, use an appropriate XML-based format (such as XHTML or OOXML), but not standard HTML5.

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:o” not allowed here.

xmlns
svg

The “xmlns:o” attribute is not allowed because it is not part of the standard HTML attributes and typically appears due to improperly embedded XML or Office-related HTML content.

In HTML documents, the “xmlns” attribute is used to declare a namespace and is generally valid for certain elements when dealing with XML documents or XHTML. However, the “xmlns:o” attribute specifically relates to XML namespaces typically used in Microsoft’s Office XML formats (for example, when copying and pasting styled content from Word to an HTML editor). Since HTML5 doesn’t natively support these namespaces as valid HTML attributes and strictly validates against them, any non-standard attributes like “xmlns:o” generate validation errors.

To fix this issue, you should remove the “xmlns:o” attribute unless you are sure that it’s necessary for the function of your document, and the document should be processed in a way that supports such namespaces. If the content is not meant to include Office-specific data elements, it’s likely this attribute was accidentally included and can be safely removed.

Example of Incorrect HTML with “xmlns:o”

<!DOCTYPE html>
<html>
<head>
  <title>Sample Document</title>
</head>
<body xmlns:o="urn:schemas-microsoft-com:office:office">
  <p>This is a paragraph within a body tag falsely including an xmlns:o attribute.</p>
</body>
</html>

Corrected Example without “xmlns:o”

<!DOCTYPE html>
<html>
<head>
  <title>Sample Document</title>
</head>
<body>
  <p>This is a paragraph within a correctly structured HTML document.</p>
</body>
</html>

By removing the “xmlns:o” attribute, the HTML document complies with the W3C standards, leading to successful validation. If you require namespaces for XML processing, it’s essential to handle them outside the context of standard HTML or within an XML or XHTML document structure where such namespaces are appropriate and valid.

Attribute “xmlns:serif” not allowed here.

Attribute “xmlns:svg” not allowed here.

svg
xmlns

The xmlns:svg attribute is not allowed on the <svg> element in HTML5 and causes validation errors.

The SVG specification only requires the xmlns (not xmlns:svg) attribute for embedding SVG in HTML, and custom namespace prefixes like xmlns:svg are not needed or permitted in HTML5. The sole required namespace declaration is xmlns="http://www.w3.org/2000/svg". The xmlns:svg form was only used in extremely outdated XML-based workflows and has no place in current HTML.

Incorrect HTML Example:

<svg xmlns="http://www.w3.org/2000/svg" xmlns:svg="http://www.w3.org/2000/svg">
  <!-- SVG content here -->
</svg>

Correct HTML Example:

<svg xmlns="http://www.w3.org/2000/svg">
  <!-- SVG content here -->
</svg>

In summary:

Use only xmlns="http://www.w3.org/2000/svg" in inline SVG within HTML documents.
Remove xmlns:svg and any other prefixed namespace attributes unless specifically required for XML workflows, not HTML.