2.1.58 [CSS-Level2-2009] Section 12.2, The 'content' property
The specification states:
'content' Value: normal | none | [ <string> | <uri> | <counter> | attr(<identifier>) | open-quote | close-quote | no-open-quote | no-close-quote ]+ | inherit Initial: normal Applies to: :before and :after pseudo-elements Inherited: no Percentages: N/A Media: all Computed value: On elements, always computes to 'normal'. On :before and :after, if 'normal' is specified, computes to 'none'. Otherwise, for URI values, the absolute URI; for attr() values, the resulting string; for other keywords, as specified.
Quirks Mode and IE7 Mode (All Versions)
The :before and :after pseudo-elements are not supported.
IE8 Mode (All Versions)
Dynamic updates to generated content do not update styles that are applied to generated content.