I made one of my own citations to experiment with the markup language, which I don't know much about. I made a nice and simple format that just lists out pretty much all the fields that could be accessible. So it has lots of lines just dumping out the formats
e.g.
<text variable="container-title" prefix=" CONTAINER-TITLE (short): " form="short"/>
<text variable="container-title" prefix=" CONTAINER-TITLE (long): " form="long"/>
<text variable="collection-title" prefix=" COLLECTION-TITLE (short): " form="short"/>
...
<date variable="issued" prefix=" ISSUED: ">
<date-part name="year" prefix=" YEAR=" />
<date-part name="month" prefix=" MONTH=" />
<date-part name="day" prefix=" DAY=" />
</date>
...
<names variable="author">
<name prefix=" AUTHOR: " delimiter=", " delimiter-precedes-last="always"/>
</names>

It gives rather ugly output like on a dummy journal entry with all fields present:

AUTHOR: First Last EDITOR: name Editor TRANSLATOR: name translator ISSUED: YEAR=2007 DAY=1 ACCESSED: YEAR=2007 MONTH=October DAY=9 CONTAINER-TITLE (short): Publication CONTAINER-TITLE (long): Publication COLLECTION-TITLE (short): Series Title COLLECTION-TITLE (long): Series Title PAGE (short): 199 PAGE (long): 199 VOLUME (short): Volume VOLUME (long): Volume ISSUE (short): Issue ISSUE (long): Issue URL (short): http://url URL (long): http://url DOI (short): doi DOI (long): doi CITATION-NUMBER (short): 1 CITATION-NUMBER (long): 1

but is useful for experimenting (if anyone wants the format let me know).

I found some interesting things - which could be down to my ignorance.

A line of the form
<text variable="abstract" prefix=" ABSTRACT: "/>
seems to break everything. It would seem to be valid according to http://xbiblio.svn.sourceforge.net/viewvc/xbiblio/csl/schema/branches/csl.rnc?view=markup
But I can't get the
chrome://zotero/content/tools/csledit.xul
to display anything with that line in, with form=short or long. Changing the abstract to abstractx means it all works again.

The line above with issued date gives the output
ISSUED: YEAR=2007 DAY=1
no matter how I try and set the month field.

There are a number of variables you can't seem to set - but that's probably fair enough. These are in the author section "publisher" "original-author" "original-publisher"
In the dates "event" and "container".
There are various variables, but that may be me not trying all formats. There are also fields in zotero entry that I cant seem to retrieve - "Reviewed author" and "contributor" for instance.