Intent to prototype: Mixed Content Auto Upgrading of display content (image, audio, video)

Summary: This security enhancing feature will automatically upgrade mixed display content from HTTP to HTTPS if the top-level document is HTTPS. Previously this would result in the mixed content indicator. Loads of type image, audio, and video will be upgraded by rewriting the URL from http: to https: without any fallback if the resource is not available over HTTPS.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1633743 <https://bugzilla.mozilla.org/show_bug.cgi?id=1633743>
Standard: https://w3c.github.io/webappsec-mixed-content/level2.html <https://w3c.github.io/webappsec-mixed-content/level2.html>
Platform coverage: All

Preference: security.mixed_content.upgrade_display_content

Devtools bug: No extra work is required for devtools.

Other browsers: Chrome has been shipping that behaviour since Chrome 81; no public signal from Apple.

web-platform-tests: There are none but we will add some within https://bugzilla.mozilla.org/show_bug.cgi?id=1673594 <https://bugzilla.mozilla.org/show_bug.cgi?id=1673594>


Cheers,
Christoph