Useful HTML Tidy configuration (formats output as XML without any HTML parsing):

doctype: omit
indent: auto
indent-spaces: 8
wrap: 100
markup: yes
output-xml: yes
input-xml: yes
tidy-mark: no

Then add the following to vimrc:

:nnoremap ,, :%!tidy -qi <Enter>

Now just press ",," to format the whole file as XML.