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For spanning all rows in a table, should we (1) use "0" as number of columns spanned, as does HTML (2) allow "all" as a much clearer value, or (3) both?
I think it is appropriate to allow both. "0" for compatibility reasons and "all" for consistency reasons.
This is a hard call. I would recommend to support "0" only, although I agree that "all" is more intuitive, but it introduces datatype inconsistency. Therefore, it would potentially force some implementations to rewrite their current logic when they parse the value of ""number-rows-spanned" and "number-columns-spanned". E.g., an old Java implementation naively uses Integer.parseInt() to convert the string to an integer, and ignore any value that's littler than 1. "0" would still work, but "all" will cause this java statement to throw a NumberFormatException, and it may not catch the exception yet in its implementation.
IMO, adding 'all' is not too much of a stretch for a XSL FO implementation. Looking at the table at http://www.w3.org/TR/xsl11/#prtab1, there are existing properties with values that are either a numeric value or a keyword, e.g., 'border-*-precedence'. 'font-size-adjelt', 'hyphenation-ladder-count', and 'z-index'.