1 files changed, 32 insertions, 7 deletions

diff --git a/src/css.js b/src/css.js
index c17265ced..7a225d9a9 100644
--- a/src/css.js
+++ b/src/css.js
@@ -28,6 +28,7 @@ var
 	// except "table", "table-cell", or "table-caption"
 	// See here for display values: https://developer.mozilla.org/en-US/docs/CSS/display
 	rdisplayswap = /^(none|table(?!-c[ea]).+)/,
+	rcustomProp = /^--/,
 	cssShow = { position: "absolute", visibility: "hidden", display: "block" },
 	cssNormalTransform = {
 		letterSpacing: "0",
@@ -57,6 +58,16 @@ function vendorPropName( name ) {
 	}
 }
 
+// Return a property mapped along what jQuery.cssProps suggests or to
+// a vendor prefixed property.
+function finalPropName( name ) {
+	var ret = jQuery.cssProps[ name ];
+	if ( !ret ) {
+		ret = jQuery.cssProps[ name ] = vendorPropName( name ) || name;
+	}
+	return ret;
+}
+
 function setPositiveNumber( elem, value, subtract ) {
 
 	// Any relative (+/-) values have already been
@@ -218,10 +229,15 @@ jQuery.extend( {
 		// Make sure that we're working with the right name
 		var ret, type, hooks,
 			origName = jQuery.camelCase( name ),
+			isCustomProp = rcustomProp.test( name ),
 			style = elem.style;
 
-		name = jQuery.cssProps[ origName ] ||
-			( jQuery.cssProps[ origName ] = vendorPropName( origName ) || origName );
+		// Make sure that we're working with the right name. We don't
+		// want to query the value if it is a CSS custom property
+		// since they are user-defined.
+		if ( !isCustomProp ) {
+			name = finalPropName( origName );
+		}
 
 		// Gets hook for the prefixed version, then unprefixed version
 		hooks = jQuery.cssHooks[ name ] || jQuery.cssHooks[ origName ];
@@ -257,7 +273,11 @@ jQuery.extend( {
 			if ( !hooks || !( "set" in hooks ) ||
 				( value = hooks.set( elem, value, extra ) ) !== undefined ) {
 
-				style[ name ] = value;
+				if ( isCustomProp ) {
+					style.setProperty( name, value );
+				} else {
+					style[ name ] = value;
+				}
 			}
 
 		} else {
@@ -276,11 +296,15 @@ jQuery.extend( {
 
 	css: function( elem, name, extra, styles ) {
 		var val, num, hooks,
-			origName = jQuery.camelCase( name );
+			origName = jQuery.camelCase( name ),
+			isCustomProp = rcustomProp.test( name );
 
-		// Make sure that we're working with the right name
-		name = jQuery.cssProps[ origName ] ||
-			( jQuery.cssProps[ origName ] = vendorPropName( origName ) || origName );
+		// Make sure that we're working with the right name. We don't
+		// want to modify the value if it is a CSS custom property
+		// since they are user-defined.
+		if ( !isCustomProp ) {
+			name = finalPropName( origName );
+		}
 
 		// Try prefixed name followed by the unprefixed name
 		hooks = jQuery.cssHooks[ name ] || jQuery.cssHooks[ origName ];
@@ -305,6 +329,7 @@ jQuery.extend( {
 			num = parseFloat( val );
 			return extra === true || isFinite( num ) ? num || 0 : val;
 		}
+
 		return val;
 	}
 } );