How to display a website in a Word Add-in using http

Question

I'm trying to show a website in my Word Add-in. I was trying to use the <iframe> tag, like this:

<iframe src="http://www.example.com" style="width: 100%; height: 100%;" />

Although it can be shown when I directly open my home.html file in browser, it can not be displayed in my sideloaded add-in.

However, if I simply add an 's' in my url

<iframe src="https://www.example.com" style="width: 100%; height: 100%;" />

then it works fine, So I think the point is the http and https. But I have not found any description in the documentation said the Add-in only supports https and do not supports http url.

I have tried adding the URL to the <AppDomain> tag in my .xml file, but it did not fix the issue.

So I wonder how can I load the http website in my addin? Are there any options in Word to enable http domains？