Basically, I am interested in knowing what prevents a resource from being saved locally, but while still allowing the browser to load it?
One way they might be doing this is to look at the http referer header. This trick is often used in hotlink protection and the theory goes something like this:
- Browser opens webpage.
- Browser fetches resources, sending referrer value of
^http://www.site.com/ (url beginning with).
- Server checks this and serves the resources if valid.
- If not, it serves a 404, or some other such error.
This is really easy to work around - install tamper data for firefox, load a page, watch the resource requests and look at their headers. Then make a direct url request, tamper it and alter the referer to match the previous request. If it works - bam, sorted.
This is a sort of effective trick for hotlink protection, but doesn't prevent you saving the resources offline.