Thanks for the many responses to my query about <add>.  I suppose what bothers
me is that it works in 80% of the cases where I need it - and there should be
some way to make it work just with CSS.  Perhaps with CSS3 or later, one day. ;-)

I'm resistant to the idea of using an html three column table for each paragraph,
because there is no straight forward way to line the marginal gloss up with its
original position.  With a paragraph of a few lines this doesn't really matter,
but some of the paragraphs in question are hundreds of lines long.

Another solution I've been considering is the possibility of using the
same CSS to float a symbol or image to the left/right whenever there is
a marginal gloss and have the content of this gloss appear in the ALT or
xhtml title attributes and hence pop-up as a tooltip on hovering.  The
immediate problem with this is that such tooltips don't allow the parsed
rendering of their content -a s far as I am aware.  I.e. The expansions of
abbreviations are being (and for the people in question really must be)
indicated with italics, which won't work with an xhtml title or alt attribute.
I suppose one could have a 'click on this image to go to a page that has the
marginal gloss on it' type of solution which would work fine for long glosses,
but would seem ridiculous for those which are a single character. ;-)

Thanks for all your suggestions in any case,