Originally Posted by dwig
Validating the HTML by browser standards is somewhat pointless when the issue is conversion to MOBI. You're not displaying this HTML in a browser, but using a tool to build a MOBI file. The HTML must be valid by the standards of the tool you choose to use for the conversion.
EPUB does have a standard and uses XHTML. To be a valid EPUB the HTML should be valid XHTML.
It sounds like the OP needs the file in both EPUB and MOBI. It is easier to start with XHTML which easily turns into EPUB then transform that into MOBI than the other way around.
As for displaying in a browser, Thats what many EPUB readers use for rendering.