HTML Guides for xmlns
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.
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.
When graphic design tools like Affinity Designer (formerly Serif) export SVG files, they often embed custom namespace declarations such as xmlns:serif="http://www.serif.com/". These namespaces allow the editor to store its own metadata — like layer names, grouping information, or application-specific settings — inside the SVG file. While this metadata is useful if you re-open the file in the original editor, it has no meaning in a web browser.
The HTML5 specification defines a specific set of namespace attributes that are allowed in SVG elements (such as xmlns, xmlns:xlink, and xmlns:xml). Any namespace prefix not in this predefined list — like xmlns:serif, xmlns:inkscape, or xmlns:sodipodi — triggers this validation error because the HTML parser cannot serialize these attributes back into well-formed XML 1.0. This isn’t just a theoretical concern: non-serializable attributes can cause issues when the DOM is manipulated via JavaScript or when the markup is processed by XML-based tools.
Beyond the xmlns:serif declaration itself, you’ll likely find attributes in the SVG that use this namespace prefix, such as serif:id="layer1". These should also be removed since they reference a namespace the browser doesn’t understand.
How to Fix It
- Remove the xmlns:serif attribute from the <svg> element.
- Remove any attributes prefixed with serif: (e.g., serif:id) from child elements within the SVG.
- If you re-export the SVG, check your editor’s export settings — some tools offer a “clean” or “optimized” export option that strips proprietary metadata.
- Consider using an SVG optimization tool like SVGO to automatically clean up unnecessary attributes.
Examples
❌ Invalid: SVG with xmlns:serif attribute
<svg xmlns="http://www.w3.org/2000/svg"
xmlns:serif="http://www.serif.com/"
viewBox="0 0 100 100"
width="100"
height="100">
<g serif:id="Layer 1">
<circle cx="50" cy="50" r="40" fill="blue" />
</g>
</svg>This triggers the error because xmlns:serif is not a recognized namespace in HTML5, and serif:id references that unsupported namespace.
✅ Valid: SVG with proprietary attributes removed
<svg xmlns="http://www.w3.org/2000/svg"
viewBox="0 0 100 100"
width="100"
height="100">
<g>
<circle cx="50" cy="50" r="40" fill="blue" />
</g>
</svg>Both xmlns:serif and serif:id have been removed. The SVG renders identically in the browser since those attributes were only meaningful to the editing application.
Handling Multiple Proprietary Namespaces
Exported SVGs sometimes contain several non-standard namespaces at once. Remove all of them:
<!-- ❌ Invalid: multiple proprietary namespaces -->
<svg xmlns="http://www.w3.org/2000/svg"
xmlns:serif="http://www.serif.com/"
xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
viewBox="0 0 200 200">
<rect x="10" y="10" width="180" height="180" fill="red"
serif:id="background"
inkscape:label="bg-rect" />
</svg>
<!-- ✅ Valid: all proprietary namespaces and prefixed attributes removed -->
<svg xmlns="http://www.w3.org/2000/svg"
viewBox="0 0 200 200">
<rect x="10" y="10" width="180" height="180" fill="red" />
</svg>If you frequently work with SVGs from design tools, integrating an SVG optimizer into your build process can save time and ensure these non-standard attributes never reach production.
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.
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.
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.
In XML and XHTML, the xmlns attribute is used to declare namespaces that allow elements and attributes from different vocabularies to coexist without naming conflicts. The xmlns:o prefix specifically declares the Microsoft Office namespace (urn:schemas-microsoft-com:office:office), which enables Office-specific elements and attributes like <o:p> (Office paragraph markers) within the document.
HTML5, however, is not an XML language. The HTML5 specification only permits the xmlns attribute on the <html> element (and only with the value http://www.w3.org/1999/xhtml), along with xmlns:xlink on SVG elements. Custom namespace prefixes like xmlns:o, xmlns:v (VML), and xmlns:w (Word) are not recognized as valid attributes on any HTML5 element and will trigger validation errors.
Why This Happens
This issue most commonly occurs when:
- Content is pasted from Microsoft Word into a WYSIWYG editor or CMS. Word generates its own flavor of HTML that includes Office namespace declarations and proprietary elements.
- HTML files are exported from Office applications like Word, Excel, or Outlook. These exports produce markup heavily reliant on Office-specific namespaces.
- Email templates are built using tools that generate Office-compatible HTML, carrying namespace baggage into standard web pages.
Why It Matters
- Standards compliance: HTML5 does not support arbitrary XML namespace declarations, so these attributes make your document invalid.
- Bloated markup: Office-generated HTML often includes not just the namespace declarations but also large amounts of Office-specific elements, conditional comments, and inline styles that increase page size significantly.
- Maintenance difficulty: Office-flavored HTML is harder to read, edit, and maintain compared to clean, standards-compliant markup.
- Potential rendering issues: While browsers generally ignore unrecognized attributes, the accompanying Office-specific elements (like <o:p>) can occasionally cause unexpected spacing or layout behavior.
How to Fix It
- Remove all xmlns:o attributes from your HTML elements.
- Remove any Office-specific elements such as <o:p>, <o:SmartTagType>, or similar tags that rely on the Office namespace.
- Check for other Office namespaces like xmlns:v, xmlns:w, xmlns:m, and xmlns:st1 — these should also be removed.
- Clean pasted content before inserting it into your HTML. Many text editors and CMS platforms offer a “Paste as plain text” option that strips out Office formatting.
- If you use a CMS or rich text editor, look for a “Clean up Word HTML” or similar feature to automatically strip Office artifacts.
Examples
❌ Invalid: Office namespace declarations on elements
<!DOCTYPE html>
<html lang="en">
<head>
<title>Company Report</title>
</head>
<body xmlns:o="urn:schemas-microsoft-com:office:office"
xmlns:v="urn:schemas-microsoft-com:vml"
xmlns:w="urn:schemas-microsoft-com:office:word">
<h1>Quarterly Report</h1>
<p class="MsoNormal">Revenue increased by 15% this quarter.<o:p></o:p></p>
<p class="MsoNormal">Expenses remained stable.<o:p></o:p></p>
</body>
</html>This markup contains three Office namespace declarations on the <body> element and uses <o:p> elements inside paragraphs — all of which are invalid in HTML5.
✅ Valid: Clean HTML without Office namespaces
<!DOCTYPE html>
<html lang="en">
<head>
<title>Company Report</title>
</head>
<body>
<h1>Quarterly Report</h1>
<p>Revenue increased by 15% this quarter.</p>
<p>Expenses remained stable.</p>
</body>
</html>❌ Invalid: Namespace on a div element
<div xmlns:o="urn:schemas-microsoft-com:office:office">
<p class="MsoNormal">Meeting notes from Tuesday.<o:p></o:p></p>
</div>✅ Valid: Clean version
<div>
<p>Meeting notes from Tuesday.</p>
</div>Notice that in the corrected examples, the class="MsoNormal" attribute was also removed. While MsoNormal is technically a valid class name that won’t cause a validation error, it’s an Office-generated class with no purpose unless you have corresponding CSS rules — removing it keeps your markup clean.
If you genuinely need XML namespace support for document processing, use a proper XHTML document served with the application/xhtml+xml content type, or handle the XML processing separately from your web-facing HTML.
In HTML5, the only namespace attributes recognized on SVG elements are xmlns (for the SVG namespace) and xmlns:xlink (for XLink, though xlink is now largely deprecated in favor of plain attributes). Any other custom namespace declarations — such as xmlns:serif, xmlns:inkscape, or xmlns:sketch — are not permitted by the HTML specification and will trigger a validation error.
Design tools like Affinity Designer, Adobe Illustrator, Inkscape, and Sketch often embed proprietary namespace declarations and metadata attributes in exported SVG files. These serve as internal markers for the design application (for example, to preserve layer names or editor-specific settings) but have no meaning in a web browser. Browsers simply ignore them, so they add unnecessary bytes to your page without providing any benefit.
Removing these attributes is important for several reasons:
- Standards compliance: The HTML5 parser has a fixed list of allowed namespace declarations. Custom ones violate the spec.
- Clean markup: Proprietary metadata bloats your SVG files with information that only matters inside the originating design tool.
- Maintainability: Removing tool-specific artifacts makes your SVGs easier to read, edit, and optimize.
To fix this, open your SVG file (or the HTML file containing inline SVGs) and remove the xmlns:serif attribute from the <svg> element. You should also search for and remove any attributes prefixed with serif: (such as serif:id) throughout the SVG, since those depend on the now-removed namespace declaration.
For a more automated approach, consider using SVGO or similar SVG optimization tools, which strip out editor metadata and unnecessary namespace declarations by default.
Examples
Incorrect — custom namespace attribute present
<svg xmlns="http://www.w3.org/2000/svg"
xmlns:serif="https://www.serif.com/"
viewBox="0 0 100 100">
<circle serif:id="MainCircle" cx="50" cy="50" r="40" fill="blue" />
</svg>This triggers two types of errors: xmlns:serif is not allowed on the <svg> element, and serif:id is not a recognized attribute on <circle>.
Correct — custom namespace and prefixed attributes removed
<svg xmlns="http://www.w3.org/2000/svg"
viewBox="0 0 100 100">
<circle cx="50" cy="50" r="40" fill="blue" />
</svg>Incorrect — multiple proprietary namespaces
Design tools sometimes add several custom namespaces at once:
<svg xmlns="http://www.w3.org/2000/svg"
xmlns:serif="https://www.serif.com/"
xmlns:xlink="http://www.w3.org/1999/xlink"
viewBox="0 0 200 200">
<g serif:id="Layer1">
<rect x="10" y="10" width="80" height="80" fill="red" />
</g>
</svg>Correct — only standard namespaces retained
<svg xmlns="http://www.w3.org/2000/svg"
viewBox="0 0 200 200">
<g>
<rect x="10" y="10" width="80" height="80" fill="red" />
</g>
</svg>Note that xmlns:xlink was also removed in this example. While it won’t always trigger a validation error by itself, it’s unnecessary if no xlink:href attributes are used — and modern HTML5 SVG usage favors plain href over xlink:href anyway.
In HTML5, SVG elements are natively supported and don’t require explicit namespace declarations at all. When you write an <svg> tag inside an HTML document, the browser automatically associates it with the correct SVG namespace (http://www.w3.org/2000/svg). The xmlns:svg="http://www.w3.org/2000/svg" attribute is a prefixed namespace binding — it declares svg as a namespace prefix — which is a concept from XML/XHTML workflows that HTML5’s parser does not support or recognize.
The HTML specification defines a limited set of allowed attributes on each element. Since xmlns:svg is not among them for the <svg> element (or any HTML5 element), the W3C validator flags it as invalid. The only namespace-related attribute the HTML parser tolerates on <svg> is xmlns="http://www.w3.org/2000/svg", and even that is optional — it’s accepted purely for compatibility with XHTML but has no functional effect in HTML5.
This issue commonly appears when SVG code is exported from graphic editors like Inkscape or Adobe Illustrator, or when SVG files originally written as standalone XML documents are pasted directly into HTML. These tools sometimes include verbose namespace declarations that are necessary in XML contexts but invalid in HTML.
Why it matters
- Standards compliance: The HTML5 specification explicitly does not allow prefixed namespace attributes. Using them results in validation errors.
- Code cleanliness: Unnecessary attributes add clutter without any benefit, making your markup harder to read and maintain.
- Potential parsing issues: While most browsers silently ignore unrecognized attributes, relying on lenient browser behavior rather than valid markup can lead to unexpected results in edge cases or non-browser HTML consumers (e.g., email clients, content scrapers, or accessibility tools).
How to fix it
- Locate all <svg> elements in your HTML that contain the xmlns:svg attribute.
- Remove the xmlns:svg="http://www.w3.org/2000/svg" attribute entirely.
- If you’re working in an HTML5 document (not XHTML), the xmlns="http://www.w3.org/2000/svg" attribute is also optional — you can keep or remove it.
- If your SVG uses other prefixed namespace declarations like xmlns:xlink, consider replacing them with their HTML5 equivalents (e.g., use href instead of xlink:href).
Examples
Incorrect: using xmlns:svg on an SVG element
The xmlns:svg attribute triggers the validation error:
<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>Correct: removing the prefixed namespace
Remove xmlns:svg and keep only the standard attributes:
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
<circle cx="50" cy="50" r="40" fill="blue" />
</svg>Also correct: minimal inline SVG in HTML5
In HTML5, the xmlns attribute itself is optional for inline SVG, so this is the cleanest approach:
<svg viewBox="0 0 100 100">
<circle cx="50" cy="50" r="40" fill="blue" />
</svg>Cleaning up multiple unnecessary namespaces
SVG exported from editors often includes several unnecessary namespace declarations. Here’s a before-and-after of a typical cleanup:
Before (multiple invalid namespace attributes):
<svg xmlns="http://www.w3.org/2000/svg" xmlns:svg="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" viewBox="0 0 200 200">
<image xlink:href="photo.jpg" width="200" height="200" />
</svg>After (cleaned up for HTML5):
<svg viewBox="0 0 200 200">
<image href="photo.jpg" width="200" height="200" />
</svg>Note that xlink:href has been replaced with href, which is the modern standard supported by all current browsers. The xmlns:xlink declaration is no longer needed either.
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 xmlns:w attribute is not allowed on the html element in HTML5.
HTML5 only permits the xmlns attribute for declaring the default XML namespace on the root element in XML documents, such as XHTML—not in HTML. Custom XML namespaces like xmlns:w are not valid in HTML and will cause errors in validators. This attribute is typically used in Microsoft Office-generated HTML for Word-specific XML elements, which are not part of standard HTML and should be removed for web compatibility.
If you are writing standard HTML, simply remove the xmlns:w attribute from your html tag.
Incorrect example (causes the error):
<!DOCTYPE html>
<html lang="en" xmlns:w="urn:schemas-microsoft-com:office:word">
<head>
<title>Invalid HTML Example</title>
</head>
<body>
<!-- Content here -->
</body>
</html>Correct example:
<!DOCTYPE html>
<html lang="en">
<head>
<title>Valid HTML Example</title>
</head>
<body>
<!-- Content here -->
</body>
</html>To ensure W3C compliance, do not include custom XML namespace declarations in HTML5 documents.
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.
Use xmlns="http://www.w3.org/1999/xhtml", not http://www.w3.org/1999/html.
The xmlns attribute specifies the XML namespace. For XHTML documents, the only valid value is http://www.w3.org/1999/xhtml. Using http://www.w3.org/1999/html is incorrect and causes validation errors. For regular HTML5 (not XHTML), you do not need the xmlns attribute at all.
Correct XHTML Example:
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang="en">
<head>
<title>XHTML Example</title>
</head>
<body>
<p>Hello, XHTML world!</p>
</body>
</html>Correct HTML5 Example (no xmlns attribute):
<!DOCTYPE html>
<html lang="en">
<head>
<title>HTML5 Example</title>
</head>
<body>
<p>Hello, HTML5 world!</p>
</body>
</html>For XHTML, always use xmlns="http://www.w3.org/1999/xhtml". For regular HTML5, omit the xmlns attribute.
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.
Use the MathML namespace with http:// (not https://) in the xmlns value.
Explanation
The xmlns attribute defines an XML namespace. For MathML in HTML or XHTML, the only permitted namespace URI is:
- http://www.w3.org/1998/Math/MathML
Using https:// is not equivalent and fails validation. In HTML5, embedded MathML typically doesn’t set xmlns on <math>; the parser assigns the MathML namespace automatically. In XHTML or XML contexts, xmlns="http://www.w3.org/1998/Math/MathML" is required on the <math> element (or inherited from an ancestor).
HTML examples
Reproducing the issue (invalid https:// namespace)
<!DOCTYPE html>
<html lang="en">
<head>
<title>MathML Invalid Namespace</title>
</head>
<body>
<math xmlns="https://www.w3.org/1998/Math/MathML">
<mi>x</mi><mo>+</mo><mn>1</mn>
</math>
</body>
</html>Fix in HTML5 (omit xmlns; let HTML set the namespace)
<!DOCTYPE html>
<html lang="en">
<head>
<title>MathML Correct in HTML</title>
</head>
<body>
<math>
<mi>x</mi><mo>+</mo><mn>1</mn>
</math>
</body>
</html>Fix in XHTML/XML (use the exact http:// URI)
<!DOCTYPE html>
<html lang="en">
<head>
<title>MathML Correct in XHTML/XML</title>
</head>
<body>
<math xmlns="http://www.w3.org/1998/Math/MathML">
<mi>x</mi><mo>+</mo><mn>1</mn>
</math>
</body>
</html>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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