I think the main principle is grouped under the head "information
preservation" (or "information retention"): you don't throw away
anything until you can prove some-one won't need it. (Being
"conservative' in what you send means being conservative in what you
thow away, in this case.)
This does not mean that you cannot *also* send the data in a converted
format:
<width cm="33.33333">1/3m</width>
The recipient can choose which it likes. If in doubt, do both (if this
won't cause integrity problems for later editing.)
In commercial publishing systems, the most common way to handle this
situation, however, is neither to preserve the information as a fraction
nor to convert it to decimal. It is to re-express the value as integers
values against some very small common unit which is some reasonably
accurate fraction of all the bases and demonimators desired. See EMUs
(http://www.oreillynet.com/xml/blog/2007/04/what_is_an_emu.html) for
example.
Cheers
Rick Jelliffe
