Users can allow mixed content to appear. The problem that changed recently is that Chrome and Firefox stick a tiny shield in the navigation bar that a user would have to click on to allow the mixed content. Internet Explorer shows a popup warning and is therefore easier to see. Safari does not have this issue.
Of course it is a bad habit for users to allow mixed content, so the best solution is to use a streaming media server that delivers content via an encrypted connection. Fortunately we have one in Los Rios Gmail and Google Apps. For instructors who have been using the CRC streaming media server, the best practice is to upload your videos to your Los Rios Google Drive and re-share them via the embed code you can copy there and paste into a Desire2Learn content topic.
I have made a video showing how to do this: