Hey liumin,
if you include just RF4 jars (which afaik includes taglibs which JBoss Tools validates against),
then you should get validation errors for using wrong tags.
However JBoss Tools doesn't validate tag's attributes against taglib.
In fact, it can't since you can use whatever attribute you want and JSF is able to process it.
I agree that JBoss Tools could optionally validate against taglib attributes (this feature could be turn on when it causes problems).