The html client is a light weigth client for viewing urls to
annotate. The original design was a single HTML page with an IFrame
included. This desing is difficult to implement because of the Cross
Site Scripting security in the Web Browsers. This security makes it
impossible for 2 windows not loaded from the same page to interact. A
call to frames['includedFrame'].document will create a security
exception.
There are known work around this situation:
- Cross Domain Frame Communication with Fragment Identifiers
- CrossFrame Communication Mechanism Across Documents and Across
Domains(an extension from the previous using frame proxy to
communicate)

However all those implementation imply that both source pages 'know'
they will participate in cross domain communication. They need to load
JavaScript files. This situation is not acceptable for the HTML
client.

A solution would be to use a proxy to either make the browser believe
all pages come from the same site(the proxy) or to include a
JavaScript page that would allow cross domain communication such as:

<IfModule mod_proxy.c>
        #turning ProxyRequests on and allowing proxying from all may allow
        #spammers to use your proxy to send email.

        ProxyRequests On
        SetOutputFilter line-editor
        SetEnv LineEdit "text/plain;text/html;text/xml;*/*"
        LERewriteRule </body>  <script src="xss.js"/></body> [i]
        <Proxy *>
                AddDefaultCharset off
                Order deny,allow
                #Deny from all
                Allow from all
        </Proxy>
        # Enable/disable the handling of HTTP/1.1 "Via:" headers.
        # ("Full" adds the server version; "Block" removes all outgoing Via: headers)
        # Set to one of: Off | On | Full | Block

        ProxyVia On
</IfModule>

Also, such a setting is useful to create a quick model and test the
annotation JavaScript, the HTML Client would probably need to use
Flex.

References:
http://tagneto.blogspot.com/2006/06/cross-domain-frame-communication-with.html
http://www.adobe.com/devnet/air/flex/getting_started.html
http://www.julienlecomte.net/blog/2007/11/31/
