Another kind of URL that doesn't get rewritten is one mentioned in a <link> tag. (This is kind of similar to the prefetch issue because prefetches are indicated with <link> tags too.) An example, from https://twitter.com/twittersearch, is <link href="http://a1.twimg.com/a/1278529908/images/favicon.ico" rel="shortcut icon" type="image/x-icon" /> <link href="http://a1.twimg.com/a/1278529908/images/twitter_57.png" rel="apple-touch-icon" /> 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).