Demo page: HTTPS mixed content warnings and errors in browsers

[mfranzke]

I am visiting the demo site https://patternlab-handlebars-preview.netlify.app/?p=all using browsers like Opera or Safari.

Expected Behavior

The page would display without any browser warnings to the user.

Actual Behavior

Browsers display (partly annoying) warnings about mixed content on the website.

e.g. Safari console:

The page at https://patternlab-handlebars-preview.netlify.app/?p=all was allowed to display insecure content from http://via.placeholder.com/1200x800.

e.g. Opera displays a warning sign in the address bar saying that the connection to the page wouldn't be secure.

Steps to Reproduce

Open https://patternlab-handlebars-preview.netlify.app/.