I am sure, the problem that no Content-Type header is sent by your server when you use HTML5 has to do with a configuration on your server. When you run the script local, then no Content-Type header is sent. When I access your site, then the Content-Type header is sent correctly even on your HTML5 pages. But I don't want to discuss this behavior any longer. The sitemap script has now an additional parameter to handle such situations. Its "IGNORE_EMPTY_CONTENT_TYPE". Under normal conditions, I don't suggest to set this to true.