added editorial note about sets of locales for web browser implementa…

…tions needing to be fixed from version to version

spec/overview.html

@@ -72,6 +72,9 @@ <h1>Implementation Dependencies</h1>
         <em>Subsets of Unicode:</em> Some operations, such as collation, operate on strings that can include characters from the entire Unicode character set. However, both the Unicode Standard and the ECMAScript standard allow implementations to limit their functionality to subsets of the Unicode character set. In addition, locale conventions typically don't specify the desired behaviour for the entire Unicode character set, but only for those characters that are relevant for the locale. While the Unicode Collation Algorithm combines a default collation order for the entire Unicode character set with the ability to tailor for local conventions, subsets and tailorings still result in differences in behaviour.
       </li>
     </ul>
+    <emu-note>
+      The set of locales made available by ECMAScript hosts must not change as the result of user behaviour, and the set of available locales must not produce observable differences between two users using the same version of the same host on the same platform. As a result, ECMAScript hosts must not allow on-demand installation of new locales.
+    </emu-note>
 
     <emu-clause id="sec-compatibility">
       <h1>Compatibility across implementations</h1>