[css-ui] State that pointer-events: none on frame prevents hit-testing its contents

[Loirooriol]

https://drafts.csswg.org/css-ui-4/#valdef-pointer-events-none defines pointer-events: none as

Hit-testing acts as if the boxes generated by the element (see [CSS-DISPLAY-3]) were not there, effectively causing the element behind the pointer-events: none element to become the target of hit-testing based events instead.

This should make it explicit that if you have something like

<button>You can click me</button><br>
<iframe style="pointer-events: none" srcdoc="<button>You can't click me</button>"></iframe>

Then hit-testing will skip the button inside the iframe, even if it has pointer-events: auto (CSS properties don't inherit into iframes).

We already have interoperability among Blink, WebKit and Gecko. They behave as desired.

And this is important for inert: if the iframe becomes inert, it gets pointer-events: none per #6685 (comment), and this should prevent interaction with its contents, even if they aren't marked as inert.