http://www.w3.org/TR/css3-selectors/#attribute-selectors http://www.w3.org/TR/CSS21/syndata.html#value-def-identifier > identifiers (including element names, classes, and IDs in selectors) > can contain only the characters [a-zA-Z0-9] and ISO 10646 characters > U+00A0 and higher, plus the hyphen (-) and the underscore (_); they > cannot start with a digit, two hyphens, or a hyphen followed by a > digit. so "0" or "1" is not a valid identifier, must be a string and must thus be quoted. bzr revid: xmo@openerp.com-20131022142049-6fq70sielv95kmu0