What’s with: “Do you want to view only webpage content that was securely delivered”?

What's with: "Do you want to view only the webpage content that was securely delivered?" dialog box, that goes on to say:
"This web page contains content that will not be delivered using a secure https connection, which could compromise security for the entire webpage".
I have a valid SSL, yet when the webpage is viewed, with https at the beginning of the url, images are missing, etc., but when viewed, with http at the beginning of the url, the page looks as it was designed to look….
What's with: "Do you want to view only webpage content that was securely delivered"?

See original post by chrisj