title: Negative margin not giving desired effect
body: <p>I have just inserted a slider in to a site, with a negative top margin of -13px on the wrapper div so that it is slightly underneath a semi-transparent white bar (which has higher z-index) and in Opera it looks fine.</p>
  
  <p>However in Firefox, IE and Chrome, the negative margin doesn't seem to have an effect and the semi-transparent white bar is just showing the white background behind, if I remove "overflow: hidden;" from the wrapper div then the slider moves up but this breaks the actual slider re-positioning the slides halfway across the screen etc..</p>
  
  <p>The slider is the revolution slider plugin and this is a Wordpress site (though didn't post this question in the Wordpress stackoverflow site as it's not related to Wordpress core functions, simply a CSS issue i presume?).</p>
  
  <p>Slider can be seen here: <a href="http://tinyurl.com/qjumkt8" rel="nofollow">http://tinyurl.com/qjumkt8</a></p>