HTML Guides for namespace
Learn how to identify and fix common HTML validation errors flagged by the W3C Validator — so your pages are standards-compliant and render correctly across every browser. Also check our Accessibility Guides.
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 xmlns attribute defines the XML namespace for an element. For SVG, the correct namespace is http://www.w3.org/2000/svg, declared with xmlns="http://www.w3.org/2000/svg". The xmlns:svg attribute attempts to declare an additional prefixed namespace binding — essentially mapping the prefix svg: to the same namespace URI. This is redundant because the default (unprefixed) namespace already covers all SVG elements.
In HTML5, the parser handles namespaces internally. The HTML specification only permits a small set of namespace attributes: xmlns on certain elements (like <svg> and <math>) and xmlns:xlink for legacy compatibility. Arbitrary prefixed namespace declarations like xmlns:svg are not part of the HTML serialization format. The W3C validator raises this error because attributes containing colons in their local names (other than the specifically allowed ones) cannot be round-tripped through the HTML parser and serializer — they are “not serializable as XML 1.0.”
Why this matters
- Standards compliance: HTML5 has strict rules about which namespace declarations are permitted. Using xmlns:svg violates these rules.
- Serialization issues: If a browser parses the HTML and then re-serializes it (e.g., via innerHTML), the xmlns:svg attribute may be lost, altered, or cause unexpected behavior because it falls outside the serializable attribute set.
- Redundancy: Even in pure XML/SVG documents, declaring xmlns:svg="http://www.w3.org/2000/svg" alongside xmlns="http://www.w3.org/2000/svg" is unnecessary. The default namespace already applies to the <svg> element and all its unprefixed descendants.
How to fix it
- Locate the <svg> element (or any element) that contains the xmlns:svg attribute.
- Remove xmlns:svg="http://www.w3.org/2000/svg" entirely.
- Ensure the standard xmlns="http://www.w3.org/2000/svg" attribute remains if needed (note that when embedding SVG inline in an HTML5 document, even xmlns is optional since the HTML parser infers the namespace automatically).
Examples
Incorrect: redundant prefixed namespace declaration
<svg xmlns="http://www.w3.org/2000/svg" xmlns:svg="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
<circle cx="50" cy="50" r="40" fill="blue" />
</svg>The xmlns:svg attribute triggers the validation error because it is not serializable in HTML.
Correct: standard namespace only
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
<circle cx="50" cy="50" r="40" fill="blue" />
</svg>Correct: inline SVG in HTML5 without any namespace attribute
When SVG is embedded directly in an HTML5 document, the HTML parser automatically assigns the correct namespace, so you can omit xmlns altogether:
<svg viewBox="0 0 100 100">
<circle cx="50" cy="50" r="40" fill="blue" />
</svg>This is perfectly valid and is the most common pattern for inline SVG in modern HTML. The xmlns attribute is only strictly necessary when the SVG is served as a standalone XML file (with a .svg extension or an image/svg+xml content type).
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.
The xmlns attribute declares the XML namespace for an element. For SVG elements, the only permitted namespace is "http://www.w3.org/2000/svg". When this attribute is present but set to an empty string ("") or any value other than the correct namespace, the W3C validator reports an error because the browser cannot properly associate the element with the SVG specification.
In HTML5 documents (served as text/html), the xmlns attribute on <svg> is actually optional. The HTML parser automatically associates <svg> elements with the correct SVG namespace without needing an explicit declaration. However, if you do include the xmlns attribute — for example, because your SVG was exported from a design tool or copied from an XML-based source — it must contain the exact value "http://www.w3.org/2000/svg". An empty or incorrect value will cause a validation error and could lead to rendering issues in certain contexts.
This matters for several reasons:
- Standards compliance: The HTML specification explicitly restricts the allowed value for xmlns on SVG elements.
- Browser compatibility: While most modern browsers are forgiving in HTML mode, an incorrect namespace can cause problems when SVG content is used in XML contexts (such as XHTML or standalone .svg files).
- Interoperability: Tools and libraries that process your HTML may rely on the correct namespace to identify and manipulate SVG elements.
To fix this issue, you have two options:
- Set the correct value: Replace the empty or incorrect xmlns value with "http://www.w3.org/2000/svg".
- Remove the attribute entirely: Since xmlns is optional in HTML5, simply removing it is often the cleanest solution.
Examples
Incorrect: empty xmlns attribute
This triggers the validation error because the namespace value is an empty string:
<svg xmlns="" width="100" height="100">
<circle cx="50" cy="50" r="40" fill="red" />
</svg>Incorrect: wrong namespace value
Any value other than the correct SVG namespace will also trigger this error:
<svg xmlns="http://www.w3.org/2000/html" width="100" height="100">
<circle cx="50" cy="50" r="40" fill="red" />
</svg>Fix: use the correct namespace
Set xmlns to the only permitted value:
<svg xmlns="http://www.w3.org/2000/svg" width="100" height="100">
<circle cx="50" cy="50" r="40" fill="red" />
</svg>Fix: remove the xmlns attribute
In an HTML5 document, you can omit xmlns altogether since the parser handles the namespace automatically:
<svg width="100" height="100">
<circle cx="50" cy="50" r="40" fill="red" />
</svg>Note on inline SVG from external sources
Design tools like Figma, Illustrator, or Inkscape often export SVG files with the xmlns attribute already set correctly. If you’re copying SVG markup and the xmlns value gets accidentally cleared or corrupted during the process, either restore it to "http://www.w3.org/2000/svg" or remove it before embedding the SVG in your HTML. Both approaches will produce valid, working markup.
XML namespaces are identified by URI strings that act as unique names. They are never fetched or loaded by the browser — they simply serve as an identifier that must match exactly what the specification defines. The XLink namespace has been defined as http://www.w3.org/1999/xlink since its inception, and changing the protocol to https creates a completely different string that parsers and validators do not recognize.
It’s a common and understandable mistake. Developers are trained to prefer https:// URLs everywhere for security, and many linting tools or habits may encourage automatically converting http:// to https://. However, namespace URIs are a special case where this rule does not apply. The string is purely declarative — no network request is made, and no security benefit comes from using https.
It’s also worth noting that the xmlns:xlink attribute is largely obsolete in modern HTML. When SVG is embedded directly in an HTML5 document, browsers automatically handle namespace resolution. You only need xmlns:xlink when serving SVG as standalone XML (with an .svg file or application/xhtml+xml content type). In most cases, you can simply remove the attribute altogether and use xlink:href or, even better, the plain href attribute, which is now supported on SVG elements like <use>, <image>, and <a>.
Examples
Incorrect: using https:// in the namespace URI
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="https://www.w3.org/1999/xlink">
<use xlink:href="#icon-star"></use>
</svg>This triggers the validation error because https://www.w3.org/1999/xlink does not match the required namespace identifier.
Fixed: using the correct http:// namespace URI
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
<use xlink:href="#icon-star"></use>
</svg>Preferred: removing the namespace and using plain href
In HTML5, you can drop the xmlns:xlink declaration entirely and use the standard href attribute instead of xlink:href:
<svg xmlns="http://www.w3.org/2000/svg">
<use href="#icon-star"></use>
</svg>This is the cleanest approach for inline SVG in modern HTML documents. The xlink:href attribute is deprecated in SVG 2, and all modern browsers support plain href on SVG linking elements.
An XML namespace URI is a unique identifier, not an actual web address that your browser fetches. The SVG namespace was defined as http://www.w3.org/2000/svg in the original SVG specification, and that exact string is what HTML parsers and validators expect. Even though using https everywhere is a best practice for real network requests, namespace URIs are not network requests — they are simply fixed strings used to identify which XML vocabulary an element belongs to.
When you write https://www.w3.org/2000/svg instead of http://www.w3.org/2000/svg, the validator sees an unrecognized namespace. This can also cause problems in certain XML-based contexts (such as XHTML or standalone SVG files), where the browser may fail to recognize the element as SVG at all, resulting in your graphics not rendering. In standard HTML5 mode, most browsers will still render inline SVGs correctly regardless of the xmlns value, but the markup is technically invalid and may cause issues in stricter parsing environments like XML serializers, server-side renderers, or tools that process SVG as XML.
This mistake is especially common because many developers reflexively change http to https — or their editor or linter automatically does — when they see a URL-like string. The same principle applies to other namespace URIs like http://www.w3.org/1999/xhtml for HTML and http://www.w3.org/1998/Math/MathML for MathML. These are all fixed identifiers that must not be altered.
How to Fix It
Replace https:// with http:// in the xmlns attribute value. That’s it — no other changes are needed.
If your project uses automated tooling that rewrites http URLs to https, you may need to configure an exception for XML namespace URIs.
Examples
❌ Incorrect: Using https in the namespace URI
<svg xmlns="https://www.w3.org/2000/svg" viewBox="0 0 100 100">
<circle cx="50" cy="50" r="40" fill="blue" />
</svg>This triggers the validation error because https://www.w3.org/2000/svg is not a recognized namespace value.
✅ Correct: Using http in the namespace URI
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
<circle cx="50" cy="50" r="40" fill="blue" />
</svg>✅ Correct: Inline SVG in HTML without xmlns
When embedding SVG directly inside an HTML5 document, the xmlns attribute is optional — the HTML parser automatically assigns the correct namespace to <svg> elements:
<svg viewBox="0 0 100 100">
<circle cx="50" cy="50" r="40" fill="blue" />
</svg>This is perfectly valid HTML5. You only need the xmlns attribute when the SVG is served as a standalone .svg file or used within an XHTML document.
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