[1] http://www.re.be/css2xslfo/1_3_2/manual.pdf#page=23:

string-set: mark contents;

[2] CSS3 Generated and Replaced Content Module ( (2003-05-15 21:54:58 +09:00 版)) http://www.w3.org/TR/2003/WD-css3-content-20030514/#strings