Make sure that empty height/width values are still set.

src/css.js

@@ -150,12 +150,17 @@ jQuery.each(["height", "width"], function( i, name ) {
 		},
 
 		set: function( elem, value ) {
+			if ( value !== "" ) {
 				// ignore negative width and height values #1599
 				value = parseFloat(value);
 
 				if ( value >= 0 ) {
 					return value + "px";
 				}
+
+			} else {
+				return value;
+			}
 		}
 	};
 });