If the subject of the selector is a block-level element, allowed values are 'none', 'inline', 'block', and 'marker'. If the value of the 'display' has any other value, the pseudo-element will behave as if the value were 'block'.
If the subject of the selector is an inline-level element, allowed values are 'none' and 'inline'. If the value of the 'display' has any other value, the pseudo-element will behave as if the value were 'inline'.
Note. Other values may be permitted in future levels of CSS.
Saf1.2 and op7.5 both allow positioning of generated content. This nice trick uses negative margins and not positioning, though, and that even saf1.1 and moz handle nicely.
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