m.htmlsanity: don't rely on .m-transition, again.
m.htmlsanity: don't rely on .m-transition, again.

Followup to 3f7b8501c98bfead4047f397a527e096ac75e26a. Was fixed in one
place but not in the other. Also add a clarifying comment.

mosra/m.css,mosra/m.css,mosra/m.css,mosra/m.css