If you are facing this error after installing Google XML Sitemaps plugin, then its not your fault because the stylesheet linked loads without problems and the sitemap doesn't present any issues. The problem was your website is accessible at http://www.sample.com but your sitemap is browsable both at
- http://www.sample.com/sitemap.xml and
- http://sample.com/sitemap.xml
Thus, the stylesheet fails to load if you visit the sitemap without having www
So you should load your sitemap by using the following URL : http://www.sample.com/sitemap.xml