Ignoring the bad markup isn't a complete solution. Janitors
still are likely to be asked to fix it.
As to posting plain text, I've never encountered difficulty including some... as in the case of the first (untagged) para in this node, nor in the case of a single sentence of narrative reply. (alternating paras with and without tags maintains readable rendering)
Re the quality needed -- As my update, above, illustrates, coding a set of simple tests for the most egregious failures can be done even by he who frequently wears a dunce cap and without writing anything like a complete parser. If one wished to be more rigorous -- to correct ALL flaws -- then one of the html parsers should make an easy starting point.