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)

test/unit/css.js

@@ -109,11 +109,13 @@ test("css(String|Hash)", function() {
 });
 
 test("css() explicit and relative values", function() {
-	expect(9);
+	expect(27);
 	var $elem = jQuery("#nothiddendiv");
 
-	$elem.css({ width: 1, height: 1 });
+	$elem.css({ width: 1, height: 1, paddingLeft: '1px', opacity: 1 });
 	equals( $elem.width(), 1, "Initial css set or width/height works (hash)" );
+	equals( $elem.css("paddingLeft"), '1px', "Initial css set of paddingLeft works (hash)" );
+	equals( $elem.css("opacity"), '1', "Initial css set of opacity works (hash)" );
 
 	$elem.css({ width: "+=9" });
 	equals( $elem.width(), 10, "'+=9' on width (hash)" );
@@ -138,6 +140,54 @@ test("css() explicit and relative values", function() {
 
 	$elem.css( "width", "-=9px" );
 	equals( $elem.width(), 1, "'-=9px' on width (params)" );
+
+	$elem.css({ paddingLeft: "+=4" });
+	equals( $elem.css("paddingLeft"), '5px', "'+=4' on paddingLeft (hash)" );
+
+	$elem.css({ paddingLeft: "-=4" });
+	equals( $elem.css("paddingLeft"), '1px', "'-=4' on paddingLeft (hash)" );
+
+	$elem.css({ paddingLeft: "+=4px" });
+	equals( $elem.css("paddingLeft"), '5px', "'+=4px' on paddingLeft (hash)" );
+
+	$elem.css({ paddingLeft: "-=4px" });
+	equals( $elem.css("paddingLeft"), '1px', "'-=4px' on paddingLeft (hash)" );
+
+	$elem.css({ "padding-left": "+=4" });
+	equals( $elem.css("paddingLeft"), '5px', "'+=4' on padding-left (hash)" );
+
+	$elem.css({ "padding-left": "-=4" });
+	equals( $elem.css("paddingLeft"), '1px', "'-=4' on padding-left (hash)" );
+
+	$elem.css({ "padding-left": "+=4px" });
+	equals( $elem.css("paddingLeft"), '5px', "'+=4px' on padding-left (hash)" );
+
+	$elem.css({ "padding-left": "-=4px" });
+	equals( $elem.css("paddingLeft"), '1px', "'-=4px' on padding-left (hash)" );
+
+	$elem.css( "paddingLeft", "+=4" );
+	equals( $elem.css("paddingLeft"), '5px', "'+=4' on paddingLeft (params)" );
+
+	$elem.css( "paddingLeft", "-=4" );
+	equals( $elem.css("paddingLeft"), '1px', "'-=4' on paddingLeft (params)" );
+
+	$elem.css( "padding-left", "+=4px" );
+	equals( $elem.css("paddingLeft"), '5px', "'+=4px' on padding-left (params)" );
+
+	$elem.css( "padding-left", "-=4px" );
+	equals( $elem.css("paddingLeft"), '1px', "'-=4px' on padding-left (params)" );
+
+	$elem.css({ opacity: "-=0.5" });
+	equals( $elem.css("opacity"), '0.5', "'-=0.5' on opacity (hash)" );
+
+	$elem.css({ opacity: "+=0.5" });
+	equals( $elem.css("opacity"), '1', "'+=0.5' on opacity (hash)" );
+
+	$elem.css( "opacity", "-=0.5" );
+	equals( $elem.css("opacity"), '0.5', "'-=0.5' on opacity (params)" );
+
+	$elem.css( "opacity", "+=0.5" );
+	equals( $elem.css("opacity"), '1', "'+=0.5' on opacity (params)" );
 });
 
 test("css(String, Object)", function() {