iFrame not loading content from http

[SimratPal Singh]

I am running on 5.5.3 and I am using iFrame macro, where in the source is on http whereas my confluence in on https. On the page load I get a message

The page at *** was loaded over HTTPS, but requested an insecure resource ***. This request has been blocked; the content must be served over HTTPS.

[Attila Simon]

I believe you may find some hints here:

https://answers.atlassian.com/questions/219053

Based on the error message and the answers here as well the issue seems to be, that the site you are using is not https, and as such the macro will not allow it.