Upgrade HTML mixed-content meta tag
<!doctype html>
<html>
<head>
<title>whatever</title>
<meta charset="utf-8">
<meta http-equiv="Content-Security-Policy" content="upgrade-insecure-requests">
<!-- rest of header:
<link rel="stylesheet" href="something.css">
<script src="somescript.js"/>
-->
</head>
<body>
<!-- whatever -->
</body>
</html>