Currently MassMessage includes a basic HTML checker that triggers a dialog when e.g. a closing tag is potentially missing. However, the dialog stays after fixing the problem.
Maybe it is worth looking at how the Translate extension checks formatting, since it has a similar feature and error messages do disappear when you type what is missing.
Version: master
Severity: normal
I just tried locally this on the REL1_22 branch, and once I close the HTML tag, the notice goes away.
What browser/OS are you using? Are there any errors in your JavaScript console?
I see what happens. The warning dialog disappears when closing the HTML tag, but the dialog will stay if you solve the problem by deleting the HTML tag altogether.
I found this problem pasting some text containing one HTML tag, and only one because it was just a sentence of a paragraph. I didn't need any HTML formatting, so I removed the tag.
Change 97481 had a related patch set uploaded by Legoktm:
If there are no matches, null is returned
I'm not going to backport this to REL1_22 since there's nothing in master that requires 1.23 right now.