About This HTML Issue
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>Learn more:
Last reviewed: August 11, 2025
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