Thanks for the replies, everyone. I'm no XML purist, but I agree "that ain't XML".
However, the pragmatist in me believes there are circumstances that could benefit from an "XML patch" utility... Suppose I'm using multiple applications that are well-intended but not terribly well-behaved (and out of my control) that contribute markup that incorrectly nests new elements with existing elements. How do I handle the tag soup?
E.g., fairly well-defined nesting issues, as in my original note:

<a><b> this </a></b>
[download]

I know "heavily overlapped" elements are very problematic and have no straightforward solution:

<a> this <b> that </a> the other </b>
[download]

However, "trivially overlapped" elements should be much easier to handle:

<a> this <b></a> that </b>
[download]

I've begun looking at HTML-Tidy and it can handle some obvious nesting and overlap issues, so far, though it's clearly more HTML-oriented (with some XML support).

Generally, whether HTML-Tidy is able, I seek a utility that can "fix" these well-defined nesting issues (ideally, it would use given tag priorities to indicate which should be ancestors to which descendants) and trivially-overlapped elements. And if the errors are worse/unfixable, the utility gives up.
Thanks again for your indulgence, mighty monks.
- Jim W.