Identification of a "normalised" form for RASH documents

[essepuntato]

It would be good to identify a particular subset of RASH that can be used as it is for converting it in any other format without modifying the original document in any way. An example:

A formula is expressed by means of the element span with role "math" (i.e., enabling the LaTeX math environment within the RASH document), cannot be easily rendered in other non-LaTeX formats. Thus, a good normalisation here is to use only images even for expressing formulas – in this case we may miss some semantics, but we facilitate the conversion.

This issue is closely related with the #59.