ct Ct and Cd looks ugly.

Originally I use single characters in order to reduce the size of output. Now I realized that the dominating parts are fonts and images.

However we can still save a few kilo bytes from the class names.

There are two options:
 - since we have to use 2-char class names (or just ignore readability at all...), I'll rename all of them, for example `fn` for font name, `fs` for font size etc
 - use macros and just assign the letters sequentially... like const char FONT_SIZE_CLASS='a'; ...='b' etc
Maybe the 2nd one is better..