HTML Validation

Bad value “https://www.w3.org/1999/xlink” for the attribute “xmlns:link” (only “http://www.w3.org/1999/xlink” permitted here).

About This HTML Issue

The only permitted value for the xmlns:link attribute is http://www.w3.org/1999/xlink.

Although using https in the URL looks like it’s going to be more secure, in fact this URL is not used to connect it to, only to declare the namespace.

Learn more:

→ MDN: Namespaces crash course

Last reviewed: October 27, 2022

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Related W3C Validator Issues

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 “xmlns:serif” not allowed here.

Bad value “” for the attribute “xmlns” (only “http://www.w3.org/2000/svg” permitted here).

Ensure the xmlns attribute for <svg> elements is set to “http://www.w3.org/2000/svg”.

In HTML documents, the xmlns attribute in an <svg> element must be defined correctly. The xmlns attribute specifies the XML namespace for an SVG element, which should always be “http://www.w3.org/2000/svg”. If you receive a validation error indicating a bad value for xmlns, it means that the attribute is empty or incorrectly set. This namespace ensures that the SVG elements are properly recognized as SVGs by the browser and aids in maintaining proper structure and function.

Here’s a correct example of an <svg> element:

<!DOCTYPE html>
<html lang="en">
  <body>
    <svg xmlns="http://www.w3.org/2000/svg" width="100" height="100">
      <circle cx="50" cy="50" r="40" stroke="black" stroke-width="3" fill="red" />
    </svg>
  </body>
</html>

Ensure that every <svg> element you use includes the xmlns attribute with the correct value to avoid validation issues and ensure complete browser compatibility.

Bad value https://www.w3.org/1998/Math/MathML for the attribute xmlns (only http://www.w3.org/1998/Math/MathML permitted here).

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>

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