That's the answer I was looking for. I change the encoding from UTF-8 to ISO-8859-1 and the error disappears. This gives me something to ask the LaTexML folx as to why they're producing XML documents claiming to be utf8 when they aren't.

Many thanks, oh Nameless One!

Update - I even found that latexml has a --inputencoding=iso-8859-1 option to do just that. Now to figure out how to automatically detect a LaTex file's encoding ...

Update the II - checking out Encode::Guess and Encode::Detect

perl -e 'print qq(Just another Perl Hacker\n)' # where's the irony switch?