Working on a SharePoint 2010 site we had a requirement to make the site HTML5 valid. To test this we use the W3C validator on internet. After a lot of improvements we got stuck with just these kind of errors.

Attribute xmlns:ddwrt not allowed here.
Attribute with the local name xmlns:ddwrt is not serializable as XML 1.0.

This attribute was rendered in HTML that was produced by a Content Query WebPart (CQWP). A content query webpart uses several XSL files to render HTML. These XSL files were customized but there was not valid HTML any more.

In Depth

When using a XSLT stylesheet to transform a XML file you can use different XSL namespace to perform several special functions i.e. DateTime functions. Without adding these namespaces in the XSL file the functions won’t work.

<xsl:stylesheet version="1.0" 
 xmlns:x="http://www.w3.org/2001/XMLSchema"
 xmlns:d="http://schemas.microsoft.com/sharepoint/dsp"
 xmlns:cmswrt="http://schemas.microsoft.com/WebParts/v3/Publishing/runtime"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
 xmlns:msxsl="urn:schemas-microsoft-com:xslt"
 xmlns:ddwrt="http://schemas.microsoft.com/WebParts/v2/DataView/runtime">

The Problem

By just adding the namespace will also result in a namespace in the result. In my case HTML.

<h2 xmlns:ddwrt="http://schemas.microsoft.com/WebParts/v2/DataView/runtime">
    Header text
</h2>

Solution

To prevent the namespace to be rendered in the result we can add an attribute to the stylesheet element. Exclude-result-prefixes.

The value is a white-space-separated list of namespace prefixes. The namespace bound to each of the prefixes is designated as an excluded namespace. The default namespace (as declared by xmlns) may be designated as an excluded namespace by including#default in the list of namespace prefixes. The designation of a namespace as an excluded namespace is effective within the subtree of the style sheet rooted at the element bearing the exclude-result-prefixes; a subtree rooted at an <xsl:stylesheet> element does not include any style sheets imported or included by children of that <xsl:stylesheet> element.

<xsl:stylesheet version="1.0" 
 exclude-result-prefixes="x d xsl msxsl cmswrt ddwrt"
 xmlns:x="http://www.w3.org/2001/XMLSchema"
 xmlns:d="http://schemas.microsoft.com/sharepoint/dsp"
 xmlns:cmswrt="http://schemas.microsoft.com/WebParts/v3/Publishing/runtime"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
 xmlns:msxsl="urn:schemas-microsoft-com:xslt"
 xmlns:ddwrt="http://schemas.microsoft.com/WebParts/v2/DataView/runtime">

After that the HTML will be clean and look like this

<h2>Header text</h2>

For more information on the xsl:stylesheet element see MSDN or w3schools

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