I am using a custom namespace for attributes in my sitemap: http://pics.jonathancross.com/sitemap.xml
They seem to work fine.
The following attributes are part of the jcd
namespace:
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="sitemap.xsl"?>
<urlset
xmlns:jcd="http://pics.jonathancross.com"
jcd:date="2015-09-16"
xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<url jcd:imgs="0" jcd:dsize="1.3G">
<loc>http://pics.jonathancross.com/</loc>
<lastmod>2015-09-16</lastmod>
<priority>1.0</priority>
</url>
...
I also use a custom desc
element like this:
<jcd:desc>Description of page here</jcd:desc>
The element causes errors in Google Webmaster Tools, however Webmaster Tools still seems to parse the sitemap data it understands.
Motivation:Adding custom data to my sitemap means it can be used as a central database for my site. I also use xslt to render the same data into a structured table of links for humans to browse the site.