"Mixed content" refers to the scenario where a web page is served via HTTPS, but it includes assets that are served via an HTTP URL. See https://developer.mozilla.org/en-US/docs/Security/Mixed_content for some discussion of this.
In Bug 1463629 we fixed this issue for embedded iframes, by patching the HTMLPurifier core class HTMLPurifier_URIFilter_SafeIframe so that, in addition to filtering iframes for an allowed set of URLs, it also transformed them from HTTPS to HTTP if needed.
After having recently done some work on HTMLPurifier for other bugs, and becoming more familiar with their API, it now becomes apparent to me that this was a bit of a hack (patching core code should have told me this anyway). What we should have done is, instead, write up a new custom URIFilter specifically for rewriting URI's from HTTP to HTTPS in this way, and used that instead.
Doing it that way will make future HTMLPurifier upgrades easier, by eliminating the need to patch that file.
See http:// htmlpurifier. org/docs/ enduser- uri-filter. html for details about how to write a custom URI filter.