About This HTML Issue
The xmlns:v attribute declares the XML namespace urn:schemas-microsoft-com:vml, which enabled Microsoft’s proprietary Vector Markup Language in Internet Explorer versions 5 through 8. VML was a way to render vector shapes directly in HTML before SVG gained broad browser support. When working with XML-based documents (like XHTML), namespace declarations such as xmlns:v were syntactically valid. However, HTML5 is not an XML language — it has its own parsing rules and only recognizes a limited set of namespace declarations on the <html> element, specifically xmlns (for the default XHTML namespace) and xmlns:xlink (in certain SVG/MathML contexts). Any other xmlns:* attribute, including xmlns:v, xmlns:o, and xmlns:w, triggers a validation error.
Why This Is a Problem
Standards compliance: The HTML5 specification explicitly does not allow arbitrary XML namespace declarations. The W3C validator flags xmlns:v because it is not part of the HTML5 attribute set for any element.
No modern browser support: VML was only supported in Internet Explorer, which has been discontinued. No current browser renders VML content, so the namespace declaration serves no purpose.
Code cleanliness: Keeping legacy, non-functional attributes clutters your markup and can confuse developers who maintain the code. It may also cause issues with HTML parsers, linters, and build tools that enforce strict HTML5 compliance.
How to Fix It
-
Remove the
xmlns:vattribute from your<html>tag (or wherever it appears). -
Remove any related namespace attributes like
xmlns:o(Office namespace) orxmlns:w(Word namespace), which are also invalid in HTML5 and often appear alongsidexmlns:v. -
Replace VML content with SVG if your page relied on VML for vector graphics. SVG is natively supported in all modern browsers and requires no namespace declaration on the
<html>element. -
Ensure the
langattribute is set on the<html>element for accessibility, since you’re already editing that line.
Examples
Invalid: Using xmlns:v on the html element
<!DOCTYPE html>
<html xmlns:v="urn:schemas-microsoft-com:vml"
xmlns:o="urn:schemas-microsoft-com:office:office"
xmlns:w="urn:schemas-microsoft-com:office:word">
<head>
<title>My Page</title>
</head>
<body>
<p>Hello world</p>
</body>
</html>
This triggers multiple validation errors — one for each xmlns:* attribute that HTML5 does not recognize.
Valid: Namespace attributes removed
<!DOCTYPE html>
<html lang="en">
<head>
<title>My Page</title>
</head>
<body>
<p>Hello world</p>
</body>
</html>Replacing VML with SVG
If your page used VML to draw shapes, replace the VML markup with inline SVG. No namespace declaration on <html> is needed — the <svg> element carries its own namespace implicitly in HTML5.
<!DOCTYPE html>
<html lang="en">
<head>
<title>Vector Graphics with SVG</title>
</head>
<body>
<svg width="200" height="200" aria-label="Blue circle">
<circle cx="100" cy="100" r="80" fill="steelblue" />
</svg>
</body>
</html>Common Sources of This Issue
This attribute frequently appears in HTML that was:
- Exported from Microsoft Word or Outlook. These applications generate HTML with VML namespace declarations for shapes, text effects, and email formatting.
- Copied from legacy templates. Older website templates and email templates sometimes included VML for rounded corners or background images in Internet Explorer.
- Generated by outdated WYSIWYG editors that targeted older IE versions.
If you’re working with HTML email templates, note that some email clients (notably older Outlook desktop versions) still use the Word rendering engine and may rely on VML for certain effects like background images. In that context, you may intentionally keep VML in your email source — but be aware that the markup will not pass HTML5 validation. For web pages, there is no reason to retain these attributes.
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