Though mla(6) works with htmlroff(1) these files may help you use it with CSS.

clean_html is an rc(1) script to change the output of htmlroff to use change the paragraphs from using style attributes to class. tmac.la.css is an example CSS file.

You can use something like this to make a link to it:

.am HEAD
\!\<link rel="stylesheet" type="text/css" href="terms.css"\>
..

See mhtml(6) for more information.