How to set tabindex in popup?

Question

Web development

Good afternoon.
Using the web with hotkeys is not new, and there are still implementation questions.
There are many interactive elements on the page that can be cycled through with the TAB key, thus each of them will be in focus.
Let's say the 4th out of 10 is a button that calls a popup or a hidden block that also has interactive elements.
The popup opens and when you press TAB, the focus is not on the elements in it, but on those that follow the button that calls our modal window.
How to solve this problem?
The use of tabindex did not give a positive result.
Transferring focus using JS to a popup seems like a bad idea, since the popup itself is not interactive and will not be able to get focus, and assigning it to the first child element is already a crutch.
Code sketch here

Answer 1

Put a certificate, since they are free now
And redirect from http to http

Answer 2

Use some kind of let's encrypt, for example to get a free certificate.

Answer 3

Here is a great solution for Accessible Modal Dialogs -- A11ycasts #19