Accessibility (hidden text) improvements

As per latest HTML5 boilerplate code.

Author: philwareham

dist/textpattern/jquery-ui.css

@@ -14,14 +14,16 @@
  * Hide jQuery UI  text but still allow screen reader access.
  */
 .ui-helper-hidden-accessible {
-  position: absolute;
+  display: inline-block;
   width: 1px;
   height: 1px;
   margin: -1px;
   padding: 0;
   border: 0;
+  white-space: nowrap;
   overflow: hidden;
-  clip: rect(0 0 0 0);
+  -webkit-clip-path: inset(50%);
+          clip-path: inset(50%);
 }
 
 /**