I had similar question like you. But, through some search I found that the difference between HTML Meta Tags and XML or XHTML Meta Tags is a single / (slash).
XML or XHTML Meta Tags end with />, while HTML Meta Tags end only with >.
e.g.
HTML Meta Tag:
HTML Code:
<meta name="keywords" content="Clipping Path, Photoshop Masking">
XML or XHTML Meta Tag:
HTML Code:
<meta name="keywords" content="Clipping Path, Photoshop Masking" />
i.e. Each Meta Tag should be properly closed.