content security policy

Which instructs the browser to automatically upgrade any HTTP request to HTTPS.

Upgrading insecure requests

You can enable this behavior either by sending a Content-Security-Policy header with this directive:

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Content-Security-Policy: upgrade-insecure-requests

Or by embedding that same directive inline in the document’s section using a element:

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<meta http-equiv="Content-Security-Policy" content="upgrade-insecure-requests">

It is worth noting, that if the resource is not available over HTTPS, the upgraded request fails and the resource is not loaded. This maintains the security of your page.

Refer: https://goo.gl/eiWbpH