I hope to gain some performance in the likely cases: if user's keep the HTML5 style, or have no non-ascii names at all, and if users furthermore provide MIME types for all their files, then neither module would be required.

I'm also pretty sure that multiple imports come at low cost since the modules get cached. Caching is [part of the language specification](http://docs.python.org/3/reference/import.html#the-module-cache): “The module cache […] serves as a cache of all modules that have been previously imported”. Do you know of any implementation which violates this requirement?