Then, you can look at the aggregrate ruleset to find which rules have either background-image or background rules (the latter shorthand notation can include background-image specifications.)

If you want to catch everything you'd also have to keep an eye on generated content (before and after could cause the download other URLs) and list-style-image.

Also, depending on what numbers you're interested in, you might want to consider:

• The weight of the HTTP headers sent. This can easily end up being several Kb.
• Browsers/servers that support compressed content, which obviously affects the amount of data that flows over the wire.

Rather than emulating the browser you might want to consider automating one. Write a Perl W3 proxy that keeps track of the size of content that flows over it and point MSIE to it. Drive MSIE with Perl and then look at what the proxy fetched. Just a thought.