Warn if HTML uses undefined class

[adamghill]

This would be relatively easy since I have all of the data. Depends on #12 and not sure how those warnings appear? Maybe it's a flag passed into the CLI that prints errors or something to stderr?

From https://fosstodon.org/@fohrloop/112671117956558357 and https://social.data.coop/@benjaoming/112670966434072718.