Attribute “xmlns:msdt” not allowed here.

The xmlns:msdt attribute is a Microsoft Office namespace declaration, and HTML doesn't allow prefixed namespace declarations on any element.

In XML, xmlns:msdt="uuid:C2F41010-65B3-11d1-A29F-00AA00C14882" binds the prefix msdt to Microsoft's data types namespace. Office uses it to give each document property a type, with attributes like msdt:dt="string". The HTML parser doesn't process namespace declarations, so it reads xmlns:msdt as an unknown attribute that happens to have a colon in its name. The only namespace attribute HTML accepts is xmlns, with the exact value http://www.w3.org/1999/xhtml. The validator reports xmlns:msdt on any element, <html> included.

The declaration usually comes with a block of Office document properties, which SharePoint and Office write into the pages they save. The block has its own <html> and <head> tags, often with xmlns:mso next to xmlns:msdt, and keeps the properties inside a conditional comment that browsers treat as a plain comment. When the block ends up in the middle of a page, the validator also reports the stray <html> and <head> tags. Browsers drop the extra <head> tags and copy the attributes of the extra <html> tag onto the page's real <html> element.

Remove the whole block: the extra <html> and <head> tags and the comment between them. If xmlns:msdt sits on the page's own <html> tag instead, delete just the attribute. Browsers don't use the declaration or the properties, so the page looks and works the same without them. If the block comes from a SharePoint page layout, remove it from the layout so every page that uses it gets the fix. If it comes from content pasted into a web part, edit that content.

Invalid example

<div class="page-content">
  <html xmlns:mso="urn:schemas-microsoft-com:office:office" xmlns:msdt="uuid:C2F41010-65B3-11d1-A29F-00AA00C14882"><head>
  <!--[if gte mso 9]><xml>
  <mso:CustomDocumentProperties>
  <mso:ContentType msdt:dt="string">Page</mso:ContentType>
  </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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