SSL/HTTPS is an encryption method that secures the connection between the browser and the server. If you are using SSL on your site, using CDN can sometimes result in unexpected issues.
If you are using SSL on your site, all the sources are loaded over SSL. In this case, SSL must be enabled for the CDN service, otherwise your site may appear as broken due to non-loading css files. You can visit the HTTPS version of your CDN URL in your browser. If SSL has not been enabled, you will see a security warning as below.