Looks like they do it after the page loads. The home page served only harmless <iframe></iframe> - not yet uploaded insecure content. Javascript does the actual loading of the iframe .
I did some testing and I can not get the message on Chromium 18 (Linux). However, on my test page, the security icon turns green when the page loads, and then turns yellow when insecure content loads in the iframe . The same thing happens on Optimization. Therefore, it’s best to assume that this method avoids the “Download anyway” message, allowing you to download insecure content.
Do not count on this - if this is a new Chrome feature, most likely they will also understand this trick and fix it later .;)
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