Values have the following meanings:

100 to 900
    These values form an ordered sequence, where each number indicates a weight that is at least as dark as its predecessor. 
normal
    Same as '400'. 
bold
    Same as '700'. 
bolder
    Specifies the next weight that is assigned to a font that is darker than the inherited one. If there is no such weight, it simply results in the next darker numerical value (and the font remains unchanged), unless the inherited value was '900', in which case the resulting weight is also '900'. 
lighter
    Specifies the next weight that is assigned to a font that is lighter than the inherited one. If there is no such weight, it simply results in the next lighter numerical value (and the font remains unchanged), unless the inherited value was '100', in which case the resulting weight is also '100'. 