Perl (See also functional-perl TODO.)
Handle tag and attribute names safely? For example:
PXML::Element->new("<>", {"/" => 1}, [])
will currently serialize to something like <<> /="1">>
use HTML::Element, as base class? Of course that won't work for general XML. XML::LibXML for the latter? Well.. Perhaps parametrizable?
how are the rules with regards to URL escaping? No escaping, right? So should actually be fine? Check in detail, tests.
tests are scattered to functional_XML/{test,testlazy}, functional_XML/t/*, and lib/PXML/Preserialize/t.pm and lib/PXML/Serialize/t.pm, which is probably too much of a mess.
clean up PXML::Serialize, it's an awful mess now (undo all those useless constant optimizations)
make a proper hierarchy (PXML::Element and PXML::Body (in PXML) should probably have a common base class), move code to proper locations.
optimization: examine whether it would be worthwhile to use mapping functions that reuse inputs if unchanged