Attribute “xmlns:style” not allowed here.

The xmlns:style attribute declares an XML namespace prefix, and HTML elements don't accept namespace declarations other than xmlns itself.

In XML, xmlns:style="..." binds the prefix style to a namespace URI, so the document can use names like style:font. The HTML parser doesn't process namespace declarations. It reads xmlns:style as an ordinary attribute with an unusual name and does nothing with it. The one namespace attribute an HTML element may have is xmlns, and only with the exact value http://www.w3.org/1999/xhtml. The validator rejects prefixed declarations like xmlns:style on every HTML element, <html> included.

The attribute usually comes from markup generated with XML tools. An XSLT stylesheet, for example, copies the namespace declarations in scope onto each literal element it outputs, unless the prefix is listed in exclude-result-prefixes:

<xsl:stylesheet version="1.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:style="http://www.w3.org/1999/xhtml"
  exclude-result-prefixes="style">

When a template repeats the declaration on the same element, the validator also reports a duplicate attribute error. Removing xmlns:style fixes both, and the page renders the same because the attribute has no effect in browsers.

Invalid example

<div class="top-bar" xmlns:style="http://www.w3.org/1999/xhtml">
  <a href="/">Home</a>
</div>

Valid example

<div class="top-bar">
  <a href="/">Home</a>
</div>

Learn more:

MDN Reference

MDN Web Docs: HTML global attributes

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WHATWG Spec

HTML Standard: Semantics, structure, and APIs of HTML documents

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W3C Reference

W3C: XSL Transformations (XSLT) Version 1.0

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