When embedding a tweet that contains a video the Twitter Widget also renders the profile picture in the bottom left of the video. This profile picture has an HTTP URL, even when both the page and widget use HTTPS (even the video preview image is HTTPS). This causes the browser to report page as not secure due to mixed content.
The bug is apparent by viewing the browser notices on the example page at https://dev.twitter.com/web/embedded-video/parameters3
I repasted the text above from an older topic about this that possibly was overlooked. It is still an issue from my testing today. Bug: Mixed Content in Embedded Tweet with Video
I uploaded a photo depicting the exact issue.
Thanks for your time and keep up the awesome work!