One thing which I planned but haven't done: A doctype is hard-coded in other places than each template. But most places are not really important, except one: In the 'xhtml' case of act_export() in ./inc/actions.php. What should we do about that?
Initially I thought of something like tpl_doctype() where we could set all doctypes to a choice of certain pre-defined strings. Or maybe we could create a new template file (like doctype.php) which gets called every time a doctype is needed!?
Or should we just set every other than the templates' files to have a "HTML5" doctype? But then we would also need to decide over the issue of XHTML or HTML and certain <html> and <meta> attributes...
After talking to Andi, I just changed all the doctypes (including the one in act_export()) to html5.