[css-selectors] Should iframe elements match :focus or :focus-visible?

[josepharhar]

Currently, iframe elements don't match :focus or :focus-visible in chrome, firefox, or safari. This got pointed out in a crbug: https://issues.chromium.org/issues/40944672

Here are some situations where we might or might not want to match :focus or :focus-visible

1. Clicking inside an iframe
2. Clicking a focusable element inside an iframe
3. Keyboard focusing/navigating to an iframe with no focusable content (this will just focus the iframe element)
4. Keyboard focusing/navigating to an iframe which has focusable content (this will focus the first focusable element in the iframe)