RFC Link: http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=37242

You can read the RFC post for the indepth details but basically the Backend should never output HTML, this is bad practice as it limits the control of the semantics of the code for the theme. Exspecially if the outputed code fails validation. This happens in several places I have started a list for them in the RFC I believe I got them all but the more eyes on the better...