HTML Guide for non-serializable attribute

Attribute with the local name “xmlns:o” is not serializable as XML 1.0.

Remove the xmlns:o attribute from your HTML elements.

When working with HTML documents intended for the web, often using namespaces like in XML documents may result in validation issues under specific standards like the W3C HTML validator. The xmlns:o attribute is typically associated with XML or XHTML documents and may not align with HTML5’s requirements, causing serialization issues in environments expecting XML 1.0 compliance. These attributes are often seen in documents meant to incorporate Microsoft Office-specific XML schemas—information that HTML5 doesn’t require or recognize.

For HTML5 documents, consider restructuring the content to avoid unnecessary XML namespace attributes. HTML5 has evolved away from strict reliance on XML, focusing instead on a more simplified model.

Example:

Before:

<!DOCTYPE html>
<html xmlns:o="urn:schemas-microsoft-com:office:office">
  <head>
    <title>Page Title</title>
  </head>
  <body>
    <h1>Hello World!</h1>
  </body>
</html>

After:

<!DOCTYPE html>
<html>
  <head>
    <title>Page Title</title>
  </head>
  <body>
    <h1>Hello World!</h1>
  </body>
</html>

By removing the xmlns:o attribute, the document aligns with HTML5 standards without the unnecessary constraints of XML namespaces.