So far this doesn’t exist, but we wish it did.
I would love to be able to edit XML SOX and have it saved as XML 1.0. In the emacs buffer would appear Sox, in the saved file would appear XML.
This is an example of SOX. Much easier on the eyes and easier to make well formed than XML. The XML serialization is pretty obvious and not reproduced here.
html> head> title> My Home Page body> h1> Contact Details p> I can be contacted at a> href=mailto:email@example.com this address except when on vacation.
Where would one start making a mode like this?