I've been over and over my nesting and closings . . .
. . and I'm very careful with closings, applying them even thought browsers traditionally correct for them being missing.
Now get this. If I move the edge of the browser window in a ways all the formatting becomes correct. If I move the edge back out, some of the fixed errors stay corrected and others revert to incorrect.
I will inspect my nesting and closings one more time, but don't expect to find anything.