[1] CSS Values and Units Module Level 3 ( (2011-09-06 17:51:33 +09:00 版)) <http://www.w3.org/TR/2011/WD-css3-values-20110906/>

[2] CSS Values and Units Module Level 3 ( (2012-03-08 22:56:28 +09:00 版)) <http://www.w3.org/TR/2012/WD-css3-values-20120308/>

[3] CSS Values and Units Module Level 3 ( (2012-08-28 00:22:41 +09:00 版)) <http://www.w3.org/TR/2012/CR-css3-values-20120828/>

[4] CSS Values and Units Module Level 3 ( (2013-04-04 00:50:56 +09:00 版)) <http://www.w3.org/TR/2013/CR-css3-values-20130404/>

[5] CSS Values and Units Module Level 3 ( (2013-07-26 02:13:11 +09:00 版)) <http://www.w3.org/TR/2013/CR-css3-values-20130730/>

[6] CSS Values and Units Module Level 3 ( (2013-08-03 00:34:48 +09:00 版)) <http://dev.w3.org/csswg/css-values/>