bugfix for occasional error message in hover module
batching of classlist changes to HTML and BODY tags, addresses (as best as I reasonably can) this helpful bug report:

https://bugzilla.mozilla.org/show_bug.cgi?id=801977

there will still be classes added to body or html after runtime etc, but most only take place upon user interaction (e.g. when you open the big editor, etc) - this should significantly reduce repaints