You may use either a HTML or an XHTML, depending how you have coded. As far as I can see, you have mixed up the HTML way of coding with XHTML way, which is not good. Make up your mind:
- either you use an HTML Doctype and write the non-empty tags (link, img, meta, etc) in a HTML way: <link>, <img> <meta>, etc
- or you use an XHTML Doctype and close the non-empty tags in an XHTML way: <link />, <img /> <meta/>, etc
But not both on the same document, as it is now in your site.