Well the problem isn't debugging things once it gets executed by Mail. The problem is getting Mail to execute my extension in the first place. It does it after mail comes in, so I'm dependent on some incoming mail. This wasn't the case previously, since I could always select a message and hit the "Apply Rules" menu and force my code to get re-executed immediately.

Note that in 4.2, the attributed string no longer generates a new table for each cell, so the above workaround no longer works. That doesn't mean the attributed string properly parses the HTML, though. As can be seen from the below image (which uses the same simple html as before), there seem to now be issues with alignment (both horizontal and vertical): I'll add this to my Feedback Assistant report.

Great workaround! I ported the Swift to some Objective C for our old project, but it seems to work just fine there as well. Thanks!

Thanks for the suggestion. I tried using the WebKit NSAttributedString category method and unfortunately, I get the same result. I updated the FeedBack Assistant report to reflect these findings as well.