I want to add comment tag in my manually generated XML sitemap to grouping urls like this:
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<!-- Book -->
<url>
<loc>http://example.com/Book</loc>
</url>
<!-- Notebook -->
<url>
<loc>http://example.com/Notebook</loc>
</url>
<!-- Pen -->
<url>
<loc>http://example.com/Pen</loc>
</url>
</urlset>
Is it possible?
That should be fine, in the sitemap protocol there is an example with a comment like the ones you used: http://www.sitemaps.org/protocol.html#extending
<!-- my comment -->