this is fun.
http://tutorials.jenkov.com/java-servlets/gzip-servlet-filter.html
GZip HTTP Headers
The browser includes the Accept-Encoding HTTP header in requests sent to an HTTP server (e.g. a Java web server). The content of the Accept-Encoding header tells what content encodings the browser can accept. If that header contains the value gzip in it, the browser can accept GZip compressed content. The server can then GZip compress the content sent back to the browser.
If the content sent back from the server is GZip compressed, the server includes the Content-Encoding HTTP header with the value gzip in the HTTP response. That way the browser knows that the content is GZip compressed.