Why doesn't html frame work after switching to https?

Question

Web development

An incomprehensible problem arose:

When switching to https on one of the sites, the frame stopped working, and only html files, everything is
fine with php files.

<frame src="left.php" name="leftframe" frameborder="0" scrolling="yes" id="leftframe" /> <!--Работает-->
<frame src="intro.html" name="mainframe" frameborder="0" id="mainframe" /> <!-- Не работает -->

Chrome
Site *** blocked.
FF
Firefox can't open this page

For your safety, * prevented Firefox from displaying the page because it is embedded in another site. To see this page, you need to open it in a new window.

Answer 1

Look at the request, where the frame comes from, since https allows you to request data only from reliable sources (ssl is valid), then all requests that come from http will be unavailable and blocked for security purposes
Frame implements display from other sites, right in the browser