Mixed content error vs warning

Question

What determines whether an insecure resource is blocked or loaded anyway with a warning?

Screenshot from Chrome 53:

Created by:

(function(){var d=document.getElementsByTagName('IMG');for(var i=0,l=d.length;i<l;i++){d[i].setAttribute('src','http://placekitten.com/'+d[i].width+'/'+d[i].height)};}())

Answer 1

The blocked images are those where the img has a srcset specified.

Chrome and Firefox actively block mixed content when an img has a srcset. The src here is an http resource while the srcset uses https.

Chromium Issue 402792 - Mixed content: <img> with empty srcset attribute should be active content

Mozilla Bug 1055750 - (srcset-mixed-content) Block mixed content <img srcset> and <picture>