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Say (an encrypted) hello to a more private internet.
(blog.mozilla.org)
A place to discuss the news and latest developments on the open-source browser Firefox
I think I phrased it wrong, or there is a confusion with terms.
If a page is loaded with HTTPS, then images/CSS/JS/iFrames (resources) will not load over HTTP. The resources also have to be served via HTTPS.
If a page is loaded over HTTP, then resources (images/CSS/JS/iFrames) can be loaded over HTTPS.
My objection was to the "even if a server has HTTPS, some resources will still load over HTTP"
As far as I know, this is not strictly true either. I believe most browsers currently block mixed active content like JavaScript or iframes, but will happily load images and such over HTTP (although I would not be surprised if this is changing).