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.