re: HTML_CSS
The HTML_CSS class should already be included. If you do manage to install HTML_CSS yourself, you can edit the plugin and delete everything below the line that says "everything below this line can be deleted if you have HTML_CSS"
donncha: i am not too concerned about whether they load style information in the header, but more about calling get_header() out of context as the theme devs etc would not be expecting it. for example if they put some code in header.php which relied on footer.php to "end cleanly". twas just a bit of 'defensive' coding on my part :)
anyway pulling the whole page should not be too resource intensive (it caches it until user clicks 'reset')