CSS cleanup: removed invalid CSS from core styles

Used cleaner alternatives for .ui-helper-hidden-accessible and .ui-helper-clearfix and moved .ui-helper-zfix over to a new css file (jquery.ui.ancient.css) to separate IE6-only fixes from the rest to make it easier to remove them.

themes/base/jquery.ui.all.css

@@ -9,3 +9,4 @@
  */
 @import "jquery.ui.base.css";
 @import "jquery.ui.theme.css";
+@import "jquery.ui.ancient.css";

themes/base/jquery.ui.ancient.css

@@ -0,0 +1,17 @@
+/*
+ * jQuery UI CSS for IE6 @VERSION
+ *
+ * Copyright 2011, AUTHORS.txt (http://jqueryui.com/about)
+ * Dual licensed under the MIT or GPL Version 2 licenses.
+ * http://jquery.org/license
+ */
+
+.ui-helper-zfix {
+    width: 100%;
+    height: 100%;
+    top: 0;
+    left: 0;
+    position: absolute;
+    opacity: 0;
+    filter: Alpha(Opacity=0);
+}

themes/base/jquery.ui.core.css

@@ -15,8 +15,11 @@
 }
 .ui-helper-hidden-accessible {
     position: absolute !important;
-    clip: rect(1px 1px 1px 1px);
-    clip: rect(1px,1px,1px,1px);
+    left: -99999em;
+    top: auto;
+    width: 1px;
+    height: 1px;
+    overflow: hidden;
 }
 .ui-helper-reset {
     margin: 0;
@@ -28,6 +31,12 @@
     font-size: 100%;
     list-style: none;
 }
+.ui-helper-clearfix {
+    display: inline-block;
+}
+.ui-helper-clearfix {
+    display: block;
+}
 .ui-helper-clearfix:before,
 .ui-helper-clearfix:after {
     content: "";
@@ -36,18 +45,6 @@
 .ui-helper-clearfix:after {
     clear: both;
 }
-.ui-helper-clearfix {
-    zoom: 1;
-}
-.ui-helper-zfix {
-    width: 100%;
-    height: 100%;
-    top: 0;
-    left: 0;
-    position: absolute;
-    opacity: 0;
-    filter: Alpha(Opacity=0);
-}
 
 
 /* Interaction Cues