[arspr]

Another vote for this possible check if someone can spend some time and knowledge in it. Kovid has explained why undefined css classes are not an error, but I do think that they will usually come from typos or other user errors while manually editing a book, so a warning in Check Book could be pretty handy.

(An example I happily seldom do any more: forgetting about setting up the .nw class as I explain in my post in the Saved Searches thread).