[#9489] added style to hide content from sighted users

Contributed by Romano Licker.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@7675 e93f8b46-1217-0410-a6f0-8f06a7374b81

public/stylesheets/application.css

@@ -989,3 +989,13 @@ h2 img { vertical-align:middle; }
     table.list {margin-top:0.5em;}
     table.list th, table.list td {border: 1px solid #aaa;}
 }
+
+/* Accessibility specific styles */
+.hidden-for-sighted {
+  position:absolute;
+  left:-10000px;
+  top:auto;
+  width:1px;
+  height:1px;
+  overflow:hidden;
+}