third_party/blink/web_tests/http/tests/security/mixedContent/resources/frame-with-insecure-css-resources.html - chromium/src - Git at Google

 <!DOCTYPE html>
 <html>
 <head>
 <link rel="stylesheet" href="fetch-insecure-css-resources.css">
 </head>
 <body>
 This background image should be 20px blue square.
 <script>
 document.fonts.ready.then(function() {
     if (window.opener)
         window.opener.postMessage('done', '*');
 });
 </script>
 </body>
 </html>
	<!DOCTYPE html>
	<html>
	<head>
	<link rel="stylesheet" href="fetch-insecure-css-resources.css">
	</head>
	<body>
	This background image should be 20px blue square.
	<script>
	document.fonts.ready.then(function() {
	if (window.opener)
	window.opener.postMessage('done', '*');
	});
	</script>
	</body>
	</html>