Attribute “xmlns:mso” not allowed here.

The xmlns:mso attribute is an XML namespace declaration that SharePoint writes into the pages and files it stores, and HTML doesn't accept it on any element.

In XML, xmlns:mso="urn:schemas-microsoft-com:office:office" binds the prefix mso to the Microsoft Office namespace, the same namespace Word binds to o. SharePoint uses that prefix for page properties such as the content type or the contact name, stored as mso: elements inside <mso:CustomDocumentProperties>.

HTML elements don't take namespace declarations. The parser treats xmlns:mso as an ordinary attribute that happens to have a colon in its name, and no HTML element defines an attribute by that name. The one namespace attribute an HTML element may carry is xmlns, and only with the value http://www.w3.org/1999/xhtml, so the validator reports xmlns:mso even on the page's own <html> tag.

Most of the time the attribute arrives inside a block of SharePoint properties that ends up in the page: a second <html> tag with xmlns:mso (often next to xmlns:msdt), a <head> tag, and the properties wrapped in a <!--[if gte mso 9]> conditional comment. The mso in that comment is Office's version check, unrelated to the namespace prefix, and browsers read the whole comment as a plain comment. When the block sits inside the body, the validator also reports the stray <html> and <head> tags. Browsers skip the extra <head> tags and copy the extra <html> tag's attributes onto the page's real <html> element.

Delete the whole block: the extra <html> and <head> tags and the comment between them. If xmlns:mso is on the page's own <html> tag, delete only the attribute. Browsers ignore the declaration and the properties, so removing them doesn't change how the page looks or behaves. Edit the file the block lives in. When every page built from the same page layout or master page shows it, fix that layout or master page. When only one page has it, look in the HTML file or web part content that page pulls in.

Invalid example

<div class="page-content">
  <html xmlns:mso="urn:schemas-microsoft-com:office:office"><head>
  <!--[if gte mso 9]><xml>
  <mso:CustomDocumentProperties>
  <mso:PublishingContactName>Web team</mso:PublishingContactName>
  </mso:CustomDocumentProperties>
  </xml><![endif]-->
  </head>
  <p>Our offices are open Monday to Friday.</p>
</div>

Valid example

<div class="page-content">
  <p>Our offices are open Monday to Friday.</p>
</div>

Learn more:

MDN Reference

MDN Web Docs: HTML global attributes

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MDN Reference

MDN Web Docs: The html element

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WHATWG Spec

HTML Standard: Parsing HTML documents

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