Loom generates HTML 5 output. Where attributes out of the standard should be used, they have been named according to the custom data specification.
The framework will try to produce pseudo-XML code where possible, but it will NOT produce XHTML-compliant code. For details about why XHTML is evil (at least, with current implementations) see:
http://hixie.ch/advocacy/xhtml
http://www.webdevout.net/articles/beware-of-xhtml
http://en.wikipedia.org/wiki/XHTML
IE 6 and 7 do not support XHTML: http://blogs.msdn.com/ie/archive/2005/09/15/467901.aspx
You should also keep in mind that any error inside an XHTML document will render a blank page in the browser.