The JavaFX 2.0 css reference allows for numeric values for -fx-font-weight:
  The font's weight, using the following syntax:
  [ normal | bold | bolder | lighter | 100 | 200 | 300 | 400 | 500 | 600 | 700 | 800 | 900 ]
(the above from: http://download.oracle.com/javafx/2.0/api/javafx/scene/doc-files/cssref.html#typefont)

Numeric values will cause css parse exceptions.
Bolder and lighter don't generate exceptions, but they do anything either (but that might just be the fonts I tried).
It seems that bolder should generate a bold font if a bold font is available (which is what html css seems to do - http://www.w3schools.com/cssref/playit.asp?filename=playcss_font-weight&preval=bolder)