When using a CSS3 property that has vendor-prefixed versions available, I indent as usual with one tab and then use spaces to line up the properties on the colon. This allows for easy editing of all ‘sibling’ properties at once using [Sublime Text’s](http://www.sublimetext.com/) multiple selection feature ((http://youtu.be/czQipWJA8EU)).
.some-selector {
-webkit-text-shadow: 2px 2px 3px rgba(0, 0, 0, 0.5);
-moz-text-shadow: 2px 2px 3px rgba(0, 0, 0, 0.5);
-o-text-shadow: 2px 2px 3px rgba(0, 0, 0, 0.5);
text-shadow: 2px 2px 3px rgba(0, 0, 0, 0.5);
}