I won’t lie, I’m not a fan. I’m really, really not a fan. But if you insist on using popups, then there are three things I expect to be able to happen. In my opinion, not utilizing these three things will really piss off your viewers.
1) A close button of some kind
An “x” in the top right, an actual “close” button… something I can click to close this popup. And this button will be visible and prominent, even on mobile.
2) Clicking outside the popup will close it
If I click outside the popup—into the backdrop of the site—I expect the popup to close.
esc will close the popup
If all else fails, clicking escape on my keyboard should also close the popup. But of course this won’t work on mobile, which is why option #1 should be there too!
If you’re going to use popups on your site, you should make them as user-friendly as possible. Don’t make it difficult for readers to navigate your site and dismiss things they may not be interested in.