wp-cli package-index page shows following error in console:

Mixed Content: The page at 'https://wp-cli.org/package-index/' was loaded over HTTPS, but requested an insecure stylesheet 'http://wp-cli.org/assets/css/fa.css'. This request has been blocked; the content must be served over HTTPS

This occurs due to url paramater in _config.yml which pointed to http://wp-cli.org site.

I modified it and changed it to https and also took this opportunity to modify other such http://wp-cli.org links to https.