Web developers are often faced with the problems associated with developing a cross-browser website or application, those developers planning to use new HTML5 and CSS3 mark-up will also have to ensure that they cater for different rending/layout engines like Webkit (Safari, Chrome) or Gecko (Firefox).

Here are a few CCS examples:

/*gradients*/

/*Firefox*/
-moz-linear-gradient (left, red, silver); 

/*Safari/Chrome*/
webkit-gradient (linear, left center, right center, from(red), to(silver));

/*shadow*/

/*Firefox*/
-moz-box-shadow: 5px 5px #818181;

/*Safari/Chrome*/
-webkit-box-shadow: 5px 5px #818181;

/*Border corners*/

/*Firefox*/
-moz-border-radius: 9px 3px 9px 3px;

/*Safari/Chrome*/
-webkit-border-radius: 3px;
-webkit-border-top-left-radius: 9px;
-webkit-border-bottom-right-radius: 9px;