Description

There are some new CSS rules and they break layouts, eg.:

:where(body .is-layout-flow) > :last-child:last-child {
	margin-block-end: 0;
}

or similar one with duplicated :first-child.

When I remove this duplicated selector with inspect element and leave only 1 occurrence, then everything works again

Step-by-step reproduction instructions

It's hard to write step-by-step for this, as you will need to use some specific themes and plugins and setup some blocks, but it's happening on more websites with a different setup, but the point is, that this is a newly added CSS, so it was working fine until now.

Screenshots, screen recording, code snippet

No response

Environment info

No response

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

No