Parsing of the pathological cases doesn't seem
specifiable, but a
simplified version probably would be.
What if we only allowed ''italic'', '''bold'''
and ''''bold italic'''',
and required a separator between consecutive markup. I.e.
''a''<s/>'''b'''
=> <i>a</i><b>b</b>;
''a'''''b''' =>
<i>a'''</a>b<b>..?
What if we didn't allow nesting, so ''italic and
'''bold''''' had to be
written as ''italic and
''<s/>''''bold''''?
That would probably go along way toward making it specifiable, without
affecting 99% of the current text.
I think it's been agreed that outright rejecting any wikitext is a bad
idea. Error messages or not, the parser has to at least try.