Change XML debug output format to valid <div> tag

This used to be output in a non-standard compliant <xmp> tag, as plain
XML output (which also lead to some HTML validators to complain about
these XML tags in an HTML document).

Now it will be the app's responsibility to render these debug information
usefully.
I don't like the inline `display: none`. In the default setting, users will turn on XML and/or ERRORS debugging and wonder why nothing shows up.