It could be
added to the html comment with preprocessor data, or perhaps
as a new box in preview mode.
In no way it is expected to appear in the page viewing, nor I am saying
that we should [[Execution by burning|burn]] (yet) people not writing
100% conformant wiki-text. Also, could be collapsed by default to not
discourage new comers. The point is that getting a better insight on how
mediawiki is "helping" would be most valuable when studying why a
[[Template:Esoteric|esoterism]] isn't working.
If we did this, we'd probably want multiple channels of output, debug
and warning at least. Warnings should only be raised when what the
user intended is clearly not happening: when duplicate id's or
nonexistent attributes are removed, for instance, or template
expansion is halted. Automatically closing tags isn't something that
should be mentioned unless the user specifically asks for that level
of nitpicking, if even then. Wikitext doesn't need to aspire to be
XML.
We could add a debug level to each warning.
Wikitext doesn't need to be strict. But the wiki being intelligent about
it is not an excuse for not writing it right the first time.
Unclosed tags is the kind of thing that could completely break the page
display if unhandled/mishandled by the tidier.
The whole thing should be optional (with several debug
levels/independent features to activate) but in whole provides a number
of benefits:
*New users can learn better wikitexting by viewing the complains.
*Experienced users can see their mistakes with it
*Advanced users can use it to debug the unexpected results.
*Developers can take advantage of it to deprecate some syntaxes
*...with the same debugging facility to log how much proposed changes
are being used.
*...and moves towards defining the official dialect
and of course it must me easily ignorable to not dicourage new contributors.
I'm seeing now bug 16038 (it has just been edited), that's the kind of
thing that could be placed on the debug output.