Package unhtml is designed to remove HTML tags from text and do minor formatting updates. It is intended to avoid changing the text very much - in particular so that it doesn't mess up formatted plain text when HTML tags are not present. This allows you to run the converter on data that you received without needing to bother checking if HTML tags are in there.
That said, the package is primarily intended to handle minor HTML snippets. It isn't a full-fledged formatter. Below are tags that are ignored, handled, and intentionally skipped.
(comments) DOCTYPE abbr acronym address area article aside audio b base basefont bdi bdo big body button canvas caption center cite code col colgroup datalist dd del details dfn dialog dir dl dt em fieldset figcaption figure font footer form header html i input ins kbd keygen label legend main map mark menu menuitem meter nav noframes noscript optgroup option output param progress q rp rt ruby s samp section select small source span strike strong sub summary sub summary sup tbody textarea tfoot thead time track tt u var video wbr
applet embed frame frameset head iframe link meta object script style title
a blockquote br div h1 to h6 hr img li ol p pre table td th tr