Landing pull request 375. Added test case to support #9237 Fixes #9237.

More Details:
 - https://github.com/jquery/jquery/pull/375
 - http://bugs.jquery.com/ticket/9237

src/css.js

@@ -96,6 +96,8 @@ jQuery.extend({
 			// convert relative number strings (+= or -=) to relative numbers. #7345
 			if ( type === "string" && rrelNum.test( value ) ) {
 				value = +value.replace( rrelNumFilter, "" ) + parseFloat( jQuery.css( elem, name ) );
+				// Fixes bug #9237
+				type = 'number';
 			}
 
 			// If a number was passed in, add 'px' to the (except for certain CSS properties)