Fix #14394: Changing style !important in webkit. Close gh-1385.

(cherry picked from commit 24e58792)

Conflicts:

	src/css.js

src/css.js

@@ -292,9 +292,12 @@ jQuery.extend({
 			// If a hook was provided, use that value, otherwise just set the specified value
 			if ( !hooks || !("set" in hooks) || (value = hooks.set( elem, value, extra )) !== undefined ) {
 
-				// Wrapped to prevent IE from throwing errors when 'invalid' values are provided
-				// Fixes bug #5509
+				// Support: IE
+				// Swallow errors from 'invalid' CSS values (#5509)
 				try {
+					// Support: Chrome, Safari
+					// Setting style to blank string required to delete "style: x !important;"
+					style[ name ] = "";
 					style[ name ] = value;
 				} catch(e) {}
 			}

test/unit/css.js

@@ -970,6 +970,12 @@ test( ":visible/:hidden selectors", function() {
 	t( "Is Hidden", "#form input:hidden", ["hidden1","hidden2"] );
 });
 
+test( "Override !important when changing styles (#14394)", function() {
+	expect( 1 );
+	var el = jQuery( "<div style='display: block !important;'></div>" ).css( "display", "none" );
+	equal( el.css( "display" ), "none", "New style replaced !important" );
+});
+
 asyncTest( "Clearing a Cloned Element's Style Shouldn't Clear the Original Element's Style (#8908)", 24, function() {
 	var baseUrl = document.location.href.replace( /([^\/]*)$/, "" ),
 	styles = [{