Try running it through a validator.  It will probably "just work".  Most software that outputs HTML does so using an XML-compatible form, or very nearly so.  The likely exceptions can usually be fixed with a simple regular expression or other substitution, e.g.
	Code:
	cat file.html | sed 's/<hr>/<hr \/>/g'  > newfile.html
 or, in English, replace <hr> with <hr />.  That's just about the only difference you're likely to run into.  That and possibly the need to add </link> closing tags if the files include any CSS or </meta> tags if the files include any meta tags.
Ah.  According to Pandoc's documentation, its html output mode is actually xhtml 1.0.  Weird.  So just add 
-t html and you should be good.