<link href> URLs not rewritten
Another kind of URL that doesn't get rewritten is one mentioned in a tag.
(This is kind of similar to the prefetch issue because prefetches are indicated with tags too.)
An example, from https://twitter.com/twittersearch, is
Even if a rule affects those URLs, they don't get rewritten, and they appear to create a mixed content warning. I'm not sure whether they get loaded, or simply ignored (I haven't checked with Wireshark).