You might want to look at HTML Tidy/libtidy (and possibly the HTML::Tidy module, which uses libtidy). If memory serves, tidy has options that will allow you to replace certain tags with CSS, which might prove useful in what you are trying to do.

Another option would be to use one of the modules for parsing the HTML document, then look for those tags/attributes you wish to replace. (I suspect the earlier option might be quicker and easier, though.)

Hope that helps.