-webkit-transition / -moz-transition vs jQuery

This is a two-part question:

Which application style takes precedence: a rule -webkit-transitionin CSS or a similar method $.css()in jQuery?

Does it make sense to include a rule -webkit-transitionin my stylesheet if I also use the method .cssin jQuery? That is, adding -webkit-transitionto make the animation more effective in Safari / Webkit browsers, if I still include the jQuery rule? (This applies to -moz-transition).

Or is it better to use only jQuery?

Thank!