CSS modifications should be captured and easily extractable from the inspector for application to the developer's server/locally-stored stylesheets.
When you say "captured", do you mean just the changes which consist of the user poking at things through the inspector (sensible, given the use case you mention), or does it include any/all dynamic style modifications (e.g., from page script as well)?
These will be changes made manually to the stylesheets through the CSS editor we'll be implementing in bug 583041. I'm focusing specifically on stylesheets for the moment because it should be fairly easy to generate a diff-like log of those changes. HTML changes will follow suit. If you're looking specifically for changes that come through JS DOM manipulation, we're hoping the Console will provide some output showing those when the DOM listener stuff makes it in.
It's worth noting that bug 585563 suggests providing an alternate way of editing properties via the style panel.
The CSS editor provided in bug 583041 provides export functionality.