Things get complicated for the browser based on content-type, cache settings and SSL/vs. nonSSL. Even compression type (gzip vs. deflate) is an issue as is page content (iframes, etc.) If you have lots of traffic and want to make sure your site works with all kinds of compression, it’s worth your while to go with an appliance like the Juniper DX. I don’t work there anymore, but it was the best box on the market for HTTP/S compression and general site acceleration. We would always be able to beat the rendering bugs when other boxes erred out.

If you don’t feel like spending a bunch of money on a commercial solution, of course there are free alternatives like mod_gzip. There are several tricky parts that can bite you though, so my advice here is to be less optimized but always working.

- Throroughly test your content on IE 6, 7 and FF. text/html is usually safe to compress, while js and css are trickier with client caching so be extra careful. XSL is usually ok, XML is iffy. Double check everything in SSL if you use it. For example IE 6 can’t support compression of XML with no caching via SSL.
- Test with cache cleared and a dirty browser cache
- Use deflate (vs. gzip) if you can, especially if your solution uses chunking vs. content-length.
- Chunking reduces Time to First Byte significantly, so use that if you can. Otherwise try to compress and cache the response. Most free solutions are pretty slow (high latency) at compression, so try to avoid latency wherever you can.
- add headers “Vary: Accept-Encoding, User-Agent” so that downstream caches don’t cache the wrong thing for the wrong browser
- Use Max-Age: to avoid the 304.

We’re using lots of compression and caching techniques on an AJAX / XML / XSL ecommerce site at http://www.cocoluna.com.

Here are our current stats:

Content Av. Object size %bytes saved
HTML 780 bytes 26%
CSS 7.7K 74%
JS 19.7K 68%
XML 3.4K 76%
XSL 10.5K 35%

The site is all SSL, so you can’t sniff the network to look at the requests but you can use browser based tools. Feel free to contact me if you want more info or specific tips.