HTML: https-form inside a http-frameset -> really insecure?

Multi tool use
HTML: https-form inside a http-frameset -> really insecure?
I am using a default http frame-forward of a domain provider to load a secure https form from my main hoster:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd"><html><head><title>die Rechnung für den Zoll :-)</title><meta name="viewport" content="width=device-width, initial-scale=1, user-scalable=no"><meta http-equiv="Content-Type" content="text/html; charset=utf-8"><meta name="description" content=""><meta name="keywords" content=""><link rel="SHORTCUT ICON" href=""></head><frameset rows="100%"><frame frameborder="0" src="https://robosoft.de/zoll"></frameset></html>
But Windows-Opera and Windows-Chrome and Windows-Firefox warn that the site is insecure and that form data can be stolen, etc.
Why ?
The frameset will have no access to the content of the frame. And that one gets fully loaded via https and the form-action then directly sends via https to my main hoster and not via the frame-forward.
(I have checked this by adding a target=_blank to tag.)
I think, that is a "bug" of all common browser. But of course, i could be wrong :-)
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