.css(), the values like +=10% aren't ignored anymore (like .animate())

src/css.js

@@ -26,7 +26,7 @@ var
 	// See here for display values: https://developer.mozilla.org/en-US/docs/CSS/display
 	rdisplayswap = /^(none|table(?!-c[ea]).+)/,
 	rnumsplit = new RegExp( "^(" + pnum + ")(.*)$", "i" ),
-	rrelNum = new RegExp( "^([+-])=(" + pnum + ")", "i" ),
+	rrelNum = new RegExp( "^([+-])=(" + pnum + ")(.*)", "i" ),
 
 	cssShow = { position: "absolute", visibility: "hidden", display: "block" },
 	cssNormalTransform = {
@@ -248,6 +248,15 @@ jQuery.extend({
 		"float": "cssFloat"
 	},
 
+	cssCastUnity: function( elem, prop, unit ) {
+		var vA = jQuery.css( elem, prop, "" );
+		if ( unit !== "px" ) {
+			jQuery.style( elem, prop, vA + unit );
+			return vA / jQuery.css( elem, prop, "" ) * vA;
+		}
+		return vA;
+	},
+
 	// Get and set the style property on a DOM Node
 	style: function( elem, name, value, extra ) {
 
@@ -273,7 +282,7 @@ jQuery.extend({
 
 			// Convert "+=" or "-=" to relative numbers (#7345)
 			if ( type === "string" && (ret = rrelNum.exec( value )) ) {
-				value = ( ret[1] + 1 ) * ret[2] + parseFloat( jQuery.css( elem, name ) );
+				value = ( ret[1] + 1 ) * ret[2] + jQuery.cssCastUnity( elem, name, ret[3] );
 				// Fixes bug #9237
 				type = "number";
 			}
@@ -285,7 +294,7 @@ jQuery.extend({
 
 			// If a number was passed in, add 'px' (except for certain CSS properties)
 			if ( type === "number" && !jQuery.cssNumber[ origName ] ) {
-				value += "px";
+				value += ret && ret[3] ? ret[3] : "px";
 			}
 
 			// Support: IE9-11+

src/effects.js

@@ -30,34 +30,10 @@ var
 
 				// Starting value computation is required for potential unit mismatches
 				start = ( jQuery.cssNumber[ prop ] || unit !== "px" && +target ) &&
-					rfxnum.exec( jQuery.css( tween.elem, prop ) ),
-				scale = 1,
-				maxIterations = 20;
+					rfxnum.exec( jQuery.css( tween.elem, prop ) );
 
 			if ( start && start[ 3 ] !== unit ) {
-				// Trust units reported by jQuery.css
-				unit = unit || start[ 3 ];
-
-				// Make sure we update the tween properties later on
-				parts = parts || [];
-
-				// Iteratively approximate from a nonzero starting point
-				start = +target || 1;
-
-				do {
-					// If previous iteration zeroed out, double until we get *something*.
-					// Use string for doubling so we don't accidentally see scale as unchanged below
-					scale = scale || ".5";
-
-					// Adjust and apply
-					start = start / scale;
-					jQuery.style( tween.elem, prop, start + unit );
-
-				// Update scale, tolerating zero or NaN from tween.cur(),
-				// break the loop if scale is unchanged or perfect, or if we've just had enough
-				} while (
-					scale !== (scale = tween.cur() / target) && scale !== 1 && --maxIterations
-				);
+				start = jQuery.cssCastUnity( tween.elem, prop, unit );
 			}
 
 			// Update tween properties