Hmm...  I ran into lots of problems with this.  I initially wanted to keep any classes that were added outside of FM to be persisted across `render`s but then realised that there was no way of telling if a class should be removed or not.

Basically in the web app we often change the classes (e.g. adding a 'loading' class on article pages) and I think blasting them all away and replacing them with the ones on the `classes` object + the module name is the only way to achieve predictable behaviour.