Attribute “xmlns:pcm” not allowed here.

The xmlns:pcm attribute is an XML namespace declaration, and HTML doesn't allow prefixed namespace declarations on any element.

In XML, xmlns:pcm="urn:PageContentManager" binds the prefix pcm to that namespace, so names like pcm:item can appear inside the element. The HTML parser doesn't process namespace declarations. It keeps xmlns:pcm as a plain attribute with a colon in its name. The only namespace attribute an HTML element may have is xmlns, with the exact value http://www.w3.org/1999/xhtml. The validator reports xmlns:pcm wherever it appears, <html> included.

The attribute shows up in pages built with XSLT, such as SharePoint web parts. When an XSLT stylesheet writes out a literal element, it copies the namespace declarations in scope onto that element, except for the XSLT namespace itself and any prefix listed in exclude-result-prefixes. If you can edit the stylesheet, add pcm to that list:

<xsl:stylesheet version="1.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:pcm="urn:PageContentManager"
  exclude-result-prefixes="pcm">

Browsers do nothing with the attribute, so removing it doesn't change how the page looks or works.

Invalid example

<div class="news-list" xmlns:pcm="urn:PageContentManager">
  <ul>
    <li><a href="/news/opening-hours">Opening hours change</a></li>
  </ul>
</div>

Valid example

<div class="news-list">
  <ul>
    <li><a href="/news/opening-hours">Opening hours change</a></li>
  </ul>
</div>

Learn more:

MDN Reference

MDN Web Docs: HTML global attributes

→

WHATWG Spec

HTML Standard: Semantics, structure, and APIs of HTML documents

→

W3C Reference

W3C: XSL Transformations (XSLT) Version 1.0

→