Attribute “X” is not serializable as XML 1.0.

The attribute xmlns:serif is not valid. Check this guide for more information on this issue.

Quote characters used for attributes can use either single quotes (') or double quotes ("), and they must be properly matched, for example:

<p class="news">...</p>

A common cause for this issue is forgetting to use the equal sign (=), so the HTML parser wrongly believes the quote forms part of the attribute name, for example:

<p class "news">this is wrong</p>

Check the syntax of the affected tag, it’s probably malformed and a < character inside has been interpreted as an attribute.

For example, this code might cause this issue:

<!-- Malformed img tag -->

<img src="photo.jpg" alt="smiling cat" < />

<!-- Fixed img tag -->

<img src="photo.jpg" alt="smiling cat" />

An invalid attribute has been found on an element. Check the affected tag to ensure attributes are well-formed, and if they are you can consider using custom data attributes.

The xmlns:dt attribute is not permitted on standard HTML elements according to the HTML specification.

HTML5 does not use XML namespaces like xmlns:dt, which are only valid in certain XML vocabularies such as XHTML or when embedding MathML or SVG. In typical HTML, attributes with xmlns or any custom XML namespace prefixes are invalid and cause validation errors.

To fix this issue, simply remove the xmlns:dt attribute from your HTML tags.
If you are using a data attribute or a custom attribute, you can use data-* attributes instead, which are allowed in HTML5.

Incorrect usage with xmlns:dt:

<div xmlns:dt="urn:schemas-microsoft-com:datatypes">
  Content here
</div>

Correct usage—attribute removed:

<div>
  Content here
</div>

If you need to store custom data, use data-* attributes:

<div data-dt="urn:schemas-microsoft-com:datatypes">
  Content here
</div>

Avoid using XML namespaces in HTML5 documents to ensure your code is standards-compliant.